/ -a. 3
UNIVERSITY OF CALIFORNIA.
^Accessions No.O .%2# .# . . . . .. Class Nc .
ROADS AND PAVEMENTS.
FRED P. SPALDING,
Assistant Professor of Civil Engineering in Cornell University*
Member A tnerican Society of Civil Engineers.
NEW YORK :
JOHN WILEY & SONS,
53 EAST TENTH STREET.
FRED P. SPALDING.
SUCCESSFUL practice in the construction of highways
must depend upon correct reasoning from elementary
principles in each instance rather than upon following
definite rules or methods of construction.
The aim of this book is to give a brief discussion,
from an engineering standpoint, of the principles in-
volved in highway work, and to outline the more im-
portant systems of construction, with a view to form-
ing a text which may serve as a basis for a systematic
study of the subject.
Details and statistics of particular examples have
for the most part been excluded as undesirable in a
book of this character. Such information is available
in many forms for those having the necessary ele-
mentary training and experience to enable them to
properly use it.
Considerable space has been given to the location
and construction of country roads, as seemed proper
in view of the present general public interest in the
matter, and the probable development of this new field
of activity in engineering work. The improvement of
our common roads must come through transferring
such work to the charge of those who make it a profes-
sion, and not through teaching the public how roads
should be constructed.
F. P. S.
ITHACA, N. Y., July, 1894.
Art. i. Object of Roads i
2. Resistance to Traction 2
3. Tractive Power of Horses 7
4. Desirability of Various Surfaces 9
5. Economic Value 1 1
6. Healthfulness 14
7. Safety 16
8. Durability 18
DRAINAGE OF STREETS AND ROADS.
Art. 9. Necessity for Drainage 22
10. Surface Drainage 23
11. Subsurface Drainage 26
12. Kinds of Soil 28
13. Types of Drains 31
14. Culverts 34
1 5. Water-breaks 42
LOCATION OF COUNTRY ROADS.
Art. 16. Considerations governing Location 43
17. Length of Road 47
Art. 18. Rise and Fall 49
19. Rate of Grade -. 51
20. Examination of Country 53
21. Placing the Line 56
22. Comparison of Routes 59
23. Changing Existing Locations 63
IMPROVEMENT OF COUNTRY ROADS.
Art. 24. Nature of Improvements 67
25. Earthwork 68
26. Drainage 72
27. Earth-road Surface 73
28. Gravel Roads 76
29. Maintenance of Country Roads 78
30. Width of Country Roads 80
31. Economic Value of Road Improvements 82
32. Systems of Road Management 85
Art. 33. Definition , 88
34. Macadam Roads 89
35. Telford Foundations 91
36. Choice of Foundation 93
37. Materials 96
38. Binding Material 101
39. Compacting 103
40. Thickness of Road Covering 105
41. Cross-section 107
42. Maintenance 107
FOUNDATIONS FOR PAVEMENTS.
Art. 43. Preparation of Road-bed no
44. Purpose of Foundation 1 1 1
Art. 45. Sand Foundation 112
46. Gravel and Broken Stone 113
47. Concrete 114
48. Brick 116
49. Sand and Plank 117
50. Depth of Foundation 117
Art. 51. Paving Brick 119
52. Tests for Paving Brick 122
53. Foundations 128
54. Construction 1 29
55. Maintenance 132
Art. 56. Asphaltum 134
57. Rock Asphalt 140
58. Asphalt Blocks 142
59. Foundations 144
60. Construction 146
61. Vulcanite or Distillate Pavement 148
62. Maintenance 149
Art. 63. Wood Blocks 151
64. Foundations 1 54
65. Construction 1 56
66. Preservation of Wood 161
67. Maintenance 164
68. Healthfulness 165
Art. 69. Stone for Pavements 168
70. Cobblestone Pavements 170
71. Belgian Blocks 172
72. Granite and Sandstone Blocks 172
73. Construction 173
74. Stone Trackways 176
Art. 75. Arrangement of City Streets 178
76. Width and Cross-section 183
77. Street Grades 187
78. Street Intersections 189
79. Footways 191
80. Curbs and Gutters 196
81. Crossings 202
82. Street-railway Track 203
83. Trees for Streets 211
84. Alleys 212
ROADS AND PAVEMENTS
ART. i. OBJECT OF ROADS.
THE primary object of a road or street is to provide
a way for travel, and for the transportation of goods
from one place to another. The facility with which
traffic may be conducted over any given road depends
upon the resistance offered to the passing of vehicles
by the surface or the grades of the road, as well as
upon the freedom of movement allowed by the width
and form of the roadway. In order that a road may
offer the least resistance to traffic, it should have as
hard and smooth a surface as possible, while affording
a good foothold to horses, and should be so located as
to give the most direct route with the least gradients.
The expediency of any proposed road construction
or improvement depends upon its desirability as affect-
ing the comfort, convenience, and health of residents of
2 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the locality, and also upon its economic value which is
largely determined by its cost and durability, as well
as upon the facility it gives for the conduct of traffic.
The problem of the highway engineer, in designing
works of this character, involves the consideration of
these various elements and their proper adjustment to
give the best results.
The kinds of road surface most commonly employed
are as follows : For the streets of cities and towns,
pavements of stone blocks, brick, asphalt, and wood ;
for suburban streets and important country roads,
macadam and gravel surfaces ; for ordinary country
roads in general, surfaces of earth or gravel.
ART. 2. RESISTANCE TO TRACTION.
The resistance to traction of a vehicle on a road
surface may be divided into three parts : axle friction,
rolling resistance, and grade resistance.
Axle friction varies with the nature of the bearing
surfaces, and for vehicles of similar construction is di-
rectly proportional to the load. It is entirely inde-
pendent of the nature of the road surface.
Rolling resistance is of two kinds : that due to irregu-
larities in the surface of the road, and that of a wheel
to rolling upon a smooth surface, sometimes called
The resistance due to an inequality in the road sur-
face, is the horizontal force necessary, at the axle, to
raise the weight upon the wheel to the height of the
obstacle to be passed. Thus (Fig. i), by the principle
of the lever, P= W.
GENERAL CONSIDERATIONS. 3
For small inequalities, this resistance will be approxi-
mately inversely as the diameter
of the wheel. The effect of small
irregularities in the surface, how-
ever, is due more to the shocks and
concussions produced by them than
to the direct lifting action of the
obstacle, and the resistance due to
uneven surface is greater at high ~
than at low velocities.
Rolling friction is probably due for the most part to
the compressibility of the surface of the road, which
permits the wheel to indent it to some extent. The
wheel is thus always forcing a wave of the surface
before it, or climbing an inclination caused by its
weight upon the road surface. This rolling friction
varies for wheels of differing diameters, being less for
large than for small wheels. The experiments of M.
Morin, in France, seemed to indicate that the resist-
ance varies inversely as the diameter. Other experi-
ments have indicated a less variation, approximately
as the square root of the diameter, while Mr. D. K.
Clark (Roads and Streets by Law and Clark ; London,
1890) concludes, from a mathematical discussion based
upon the assumption that the material of the surface
is homogeneous and the pressure proportional to the
depth of penetration, that the resistance to traction
is inversely as the cube root of the diameter of the
For practical purposes it may be considered that, for
wheels of ordinary sizes used on road vehicles, the
rolling resistances are equal to the load multiplied by a
coefficient which depends upon the nature and condi-
4 A TEXT-BOOK ON ROADS AND PAVEMENTS.
tion of the road surface, although these coefficients
are somewhat affected by the sizes of the wheels.
Many experiments have been made for the purpose
of determining the tractive force required for a given
load upon various road surfaces. The results show
somewhat wide variations, as would be expected when
the many elements that may affect them are con-
sidered. The following table shows a few average
results, which will give some idea of the relative resist-
ances of various surfaces and of the advantage to be
derived from a smooth and well-kept road surface :
TRACTIVE RESISTANCES ON VARIOUS SURFACES.
Character of Road. Resistance, Lbs. per ton.
Earth ordinary in fair condition. 125 to 140
dry and hard 60 " 100
Macadam very good 40 " 60
ordinary 60 " 80
poor 100 " 150
Granite-block pavement good. . . 25 40
ordinary 50 " 80
Asphalt pavement 15 " 25
Wooden-block pavement 20 " 30
On earth roads or smooth pavements the tractive
force is independent of the velocity ; but on rough
pavements, where concussions take place, the tractive
force increases as the speed increases.
Grade Resistances. Tractive resistance due to grade
is independent of the nature of the road surface, or of
the size of the wheels. It is equal to the load multi-
plied by the sine of the angle made by the grade with
the horizontal. Thus in Fig. 2 the tractive force P,
due to the grade, is the force necessary to prevent the
wheel from rolling down
the slope under the ac- / \ \ P
tion of the weight W, or
it is the component of
^parallel to the slope ac.
Grades are ordinarily expressed in terms of rise or
fall in feet per hundred, or as percentage of horizontal
For all ordinary cases of small inclinations ab is
approximately equal to ac, and we may take
or the tractive force necessary to overcome any grade
equals the load multiplied by the percentage of
The total tractive force necessary to haul a load up
an inclined road equals the sum of the force necessary
to haul the load upon the same surface when level, and
the force necessary to overcome the grade resistance.
Thus, if we wish to find the tractive effort necessary to
haul a load of 2 tons up a grade of 3 ft. in 100 over a
good macadam road. Taking the resistance of the
road surface when level at 60 Ibs. per ton, we have for
the total resistance
R = 2 X 60 + 4000 X rib- = 240 Ibs.
6 A TEXT-BOOK ON ROADS AND PAVEMENTS.
In going down the grade, the force due to grade
becomes a propelling force, and the tractive effort
required is the difference between the surface resist-
ance and grade force. In case the grade force be the
greater, the resulting tractive force becomes negative,
or it will be necessary to apply the force as a resistance
to prevent acceleration of the velocity in the descent.
The angle for which the tractive force required for a
given surface equals the grade resistance is called the
Angle of Repose for that surface. In the case given
above, 2 X 60 4000 X yjru- = o, or the angle of repose
for a surface whose level resistance is 60 Ibs. per ton is
a 3$ grade. If a vehicle were left standing upon that
inclination, it should remain standing with the forces
just balanced. If it were started down the grade, it
should continue to move at a uniform rate, without
the application of any other force.
In a series of experiments made by the Studebaker
Brothers Manufacturing Company (see The Engi-
neering Record for Dec. 16, 1893) upon the traction
necessary upon various surfaces with American wagons,
it was found that the width of wheel-tire has little if
any effect upon a hard surface ; that there was a small
difference in favor of the wide tire upon soft ground,
and upon sod the narrow tire would cut through where
the wide one would pass over.
The wheels used in the tests had tires if, 3, and
4 inches wide, and varied from 3 ft. 6 in. to 4 ft. 6 in.
The general results show a variation in tractive force
required, depending upon the construction of the
vehicle, of from 30 to 65 pounds per ton for a stone-
block pavement, from 120 to 175 pounds per ton for a
GENERAL CONSIDERATIONS. 7
good sand road, from 60 to 100 pounds per ton for a
gravel road, and from 240 to 325 pounds per ton on a
It was also found that the force necessary to start
the load was from 125 to 200 pounds per ton greater
than that necessary to keep it in motion ; the load
starting easier on wheels of large diameter than upon
small ones, but the diameter seemingly having less
effect upon the traction when in motion.
ART. 3. TRACTIVE POWER OF HORSES.
The loads that a horse can pull upon various road
surfaces will not necessarily be proportional to the
resistance offered by the surface to traction, as the
tractive force that the horse can exert depends upon
the foothold afforded by the surface. The ability of a
horse to exert a tractive force depends upon the
strength of the animal, upon his training for the par-
ticular work, and whether he be accustomed to the
surface upon which he is travelling. The work of dif-
ferent animals is therefore subject to considerable varia-
tions, and only very rough approximations are possible
in giving average values of the work a horse may do
under differing conditions.
The tractive force that may be exerted by a horse,
at moderate speeds, varies approximately inversely as
the rate of speed ; or, in other words, the power that a
horse can exert through any considerable time is nearly
constant for varying velocities. Thus it may be as-
sumed, as an average value, that a horse working
regularly ten hours per day can put forth a tractive
8 A TEXT-BOOK ON ROADS AND PAVEMENTS.
effort of 80 pounds at a speed of 250 feet per minute
on an ordinary level road surface.
For the power of the horse we then have
Power force X velocity = 80 X 250 = 20000 foot-lbs.
For any other rate of speed, as 200 feet per minute,
we would have 20000 -f- 200 = 100 pounds as the
tractive force exerted by the horse.
If the period of daily work be lessened, the power
that may be developed will be increased, either by in-
creasing the load or the velocity.
The tractive force that a horse is able to exert de-
creases very rapidly as the rate of inclination increases.
This is due both to the expenditure of power by the
horse in lifting his own weight up the grade, and to
the less firm footing on the inclination. The effect of
differences in the foothold afforded by various pave-
ments is very marked in the loss of tractive power
In the table below are given the loads that an
average horse may be expected to continuously haul
up different inclinations, on various road surfaces, at
slow speed. These figures, while of little value as an
absolute measure of what may be done in any par-
ticular case, are of use as a rough comparison of the
relative tractive properties of different surfaces and
grades. The effect of grades upon tractive effort will
also depend upon the condition in which the surface is
maintained, and upon the weather. Snow and ice in
winter, or the damp and muddy condition of some
pavements in wet weather, have a very considerable
effect to diminish tractive power.
LOADS IN POUNDS THAT A HORSE CAN DRAW UPON
VARIOUS SURFACES AND GRADES.
Kinds of Surface.
Rate of Grade.
Earth road good
Stone Blocks good
Asphalt clean and dry
On steep grades (more than 8 or 10 in 100) special
forms of block pavements are sometimes employed to
increase the tractive power by affording better foot-
hold to horses. Sheet asphalt is not usually employed
on grades of more than 4$. Ordinary wood blocks
and brick are used up to grades of 7% or 8$, and granite
blocks to 10%.
A horse may frequently exert for a short time a
tractive force about double that which he can exert
continuously ; hence, when short grades occur steeper
than the general grades of the road, loads may often
be taken over them much heavier than could be carried
if the steeper grade prevailed upon the road.
ART. 4. DESIRABILITY OF VARIOUS ROAD
The desirability of a road surface for any particular
use depends both upon its fitness for the service re-
quired of it and upon its durability in use.
Upon a country road, the problem of improvement
ordinarily consists simply in providing the hardest and
10 A TEXT-BOOK ON ROADS AND PAVEMENTS.
most durable surface consistent with an economical
expenditure of available funds, the object being to
lighten the cost of transportation by reducing the
resistance to traction, and to render travel easy and
Upon city streets, however, several other factors
may be of importance in the design of highway im-
The comfort both of those using the street and of
the occupants of adjoining property will be largely
affected by the freedom of the surface from noise and
The safety of the pavement in use, its effect upon
the health of residents of the locality, and its economic
value must in each case be considered.
To adjust to the best advantage these various ele-
ments, frequently quite discordant with each other, is
a matter which can only be accomplished by the exer-
cise of good judgment. Local conditions and necessi-
ties must always be considered such as the difficulties
of drainage, the availability of various materials, the
nature of the traffic to be carried, and the needs of the
business or property interests of the neighborhood.
Thus, for heavy hauling of a large city, the durability
and resistance to wear of the pavement may be the
paramount consideration ; for an office district, quiet
may be very important ; for the lighter driving of a
residence street, the elements of comfort and health-
fulness may properly be considered as of greater force
than the purely economic ones ; while in all of the
cases the necessary limitation of first cost will largely
determine what may or may not be done.
GENERAL CONSIDERATIONS. II
ART. 5. ECONOMIC VALUE.
The determination of the economic value of any
proposed road or street improvement is always a
matter of difficulty, as it embraces so many items
which cannot be exactly evaluated. The factors to be
considered in this connection are :
1. Cost of construction.
2. Cost of maintenance and repairs.
3. Cost of conducting transportation.
4. Effect upon land values or business interests.
For the purpose of comparing various pavements,
or of considering the advisability of any proposed im-
provement, we may sum the interest on the cost of
construction with the annual charges representing the
other items, and find which improvement will make the
total annual cost a minimum or annual benefit a maxi-
mum. This process, in any case in practice, simply
amounts to a use of the judgment, having properly in
view the various interests to be affected, as to what
expense may legitimately be allowed in order to secure
a certain benefit. The outlay is usually quite tangible
and easily estimated, while the advantages cannot be
directly estimated and are often overlooked. It must
not, however, be supposed that they have no financial
value, or that the pavement which can be constructed
and maintained for the least money is necessarily the
most economical to use.
77^^ cost of construction must, of course, include
everything connected with the original construction
of the road and all necessary expenses leading to tl
improvement under consideration.
The cost of maintenance and repairs includetf'an esti-
12 A TEXT-BOOK ON ROADS AND PAVEMENTS.
mate of the average cost of keeping the road in good
condition over a term of years, taking into account the
necessity of renewing the surface at the expiration of
the life of the pavement, the cost of cleaning and
sprinkling, and such minor repairs as may be neces-
sary from time to time to maintain a uniformly good
An approximate estimate upon these points may
usually be made by examining the records of the same
kind of construction under similar conditions else-
where. The cost of maintenance of each kind of
pavement varies widely in different localities and
under differing treatment, and no general rules can be
stated as to the relative costs of the various systems.
All road surfaces will require maintenance, the same
as any other class of engineering constructions sub-
jected to wear in use ; and as a rule the cost of main-
tenance will be less as the care used in keeping the
surface always clean and in good condition is greater.
The cost of transportation is affected by the nature
and condition of the road over which the traffic must
pass, both because the resistance to traction offered by
the surface determines the load that may be hauled
over it, and because the roughness of the surface
serves both to limit speed and to cause wear upon
horses, harness, and vehicles. The evaluation of these
items is a matter of difficulty, on account of the practi-
cally indeterminate nature of the data upon which they
should be based.
A rough idea of the relative cost of transportation
over different road surfaces may sometimes be obtained
by observing or estimating the extent and nature of
the traffic that is likely to pass over the road and esti-
GENERAL CONSIDERATIONS. 13
mating the cost of carrying this traffic over each sur-
face. The portion of the traffic which consists in
hauling maximum loads will be directly affected by
differences in tractive resistances ; the number of loads
necessary to move the traffic, and hence the cost,
being, for this portion, approximately proportional to
the resistance. For the lighter portion of the traffic
the greater speed with a smooth surface and easy
grades will be of value in the saving of time, although
difficult to state in money values.
The effect of a smooth surface is also very appre-
ciable in the cost of wear and tear upon horses, vehi-
cles, and harness. The value of this item is variously
estimated, and probably ranges from one to ten cents
per mile travelled.
Earth roads, in good condition, and wood pavements
seem most favorable to horses, although asphalt and
broken-stone roads are commonly considered most
advantageous as to general wear. Brick would not
differ greatly from asphalt. On earth roads in poor
condition the wear is severe, and on stone blocks it is
estimated to be three or four times as great as on
asphalt. The financial value of the saving in this wear
and tear is difficult to ascertain, but it is undoubtedly
sufficient to make it an important item in the cost of
Land Values. The effect of highway improvements
upon the value of adjoining property is dependent
upon the nature of the uses to which the property may
be put, and the extent to which various characteristics
of the road surfaces, such as dust or noise, may affect
the occupations or comfort of the occupants. A
pavement may thus sometimes have a direct effect
14 A TEXT-BOOK ON ROADS AND PAVEMENTS.
upon rental values. In general, however, the effect is
difficult to estimate, although it is commonly recog-
nized in the practice of assessing a portion of the cost
of improvements against abutting property.
The value of comfort, convenience, safety, and
healthfulness to a community, as affected by the con-
dition of their roads and streets, cannot readily be
stated in figures; but they have a money value, both
in their effect upon the general life and business of the
community and in the attraction presented to outside
business enterprises or home-seekers.
ART. 6. HEALTHFULNESS.
The effect of a pavement upon the health of the
residents of its locality will be affected by the tendency
of the materials composing it to decay, by its permea-
bility, and by its degree of freedom from noise and
The permeability of a road surface is important on
account of the tendency of surface-water and refuse
matter to penetrate and saturate it, and thus cause it
to become dangerous to health. A continuous sheet
pavement is the most desirable in this particular, and
a block pavement with open joints the least so. When,
however, the joints of a block pavement are properly
cemented, the pavement may be made nearly imper-
vious. If the material of which the pavement is com-
posed be permeable, it may gradually become saturated
with street refuse, even though the joints be made
tight, and where the material is liable to decay it may
of itself become obnoxious to health.
Both these objections are raised to the use of wood
GENERAL CONSIDERATIONS. 15
pavements, and probably in many cases with justice.
This is a matter, however, concerning which authorities
differ. The extent of the danger to health involved
in the use of wood for pavements in any particular
case probably depends largely upon the wood selected
for use, and the method of construction adopted. It
is at least questionable whether the permeability of the
material used for pavements is in practice ever as ob-
jectionable on the ground of health as that caused
by open-joint construction of block pavement, even
though the material of the blocks be impervious to
The noise made by traffic upon a pavement is impor-
tant not only because of its effect upon the comfort of
the people using it or living adjacent to it, but also
because to it are frequently attributed many nervous
disorders to which people in some cities are subject.
Stone-block pavements are the most objectionable
in this particular, causing a continual roar, due both to
the rumbling of wheels over them and the blows of the
horses' feet upon them. Upon asphalt the noise is only
that due to the horses' feet, giving a sharp, clicking
sound. Upon wood the horses produce no appreci-
able sound ; but wheels give a dull rumble, generally
considered the least objectionable of any of the noises
made by the more common pavements. The noise of
wood pavements is diminished by making the joints
between blocks small. A brick surface gives a combi-
nation of the sounds of wood and asphalt, the clicking
being much less sharp than on asphalt, and the rumble
less noticeable than on wood. On any block pave-
ment the noise is lessened as the foundation is made
more firm and the joints more close and well cemented.
l6 A TEXT-BOOK ON ROADS AND PAVEMENTS.
An earth or broken-stone road is usually less noisy
than any of the hard pavements.
The giving off of dust by a pavement under the
action of traffic is also objectionable on the score of
health as well as of comfort. All pavements produce
more or less dust, the amount depending more upon
the method of construction and care used in forming
the surface and filling the joints than upon the material
of the pavements. For the most part, however, the
presence of dust is dependent rather upon the main-
tenance, cleaning, and sprinkling of the pavement than
upon its nature, and the dirt upon the surface of a
hard pavement is usually carried there from the out-
side and not due to the pavement.
Earth and broken-stone roads wear rapidly, and
make dust freely in dry weather, requiring frequent
sprinkling and cleaning to keep the road clear of it,
and are on this account objectionable for use on the
streets of towns under any considerable traffic.
ART. 7. SAFETY.
The safety of a road surface depends upon the foot-
hold afforded by it to horses under normal conditions,
and also upon the degree of slipperiness that it may
take in wet weather, or under the influence of ice and
snow in winter.
A dry earth road in good condition gives the best
and surest foothold, with broken-stone and gravel
roads nearly as good.
The relative safety of the various pavements used in
city streets is a matter upon which there is consider-
able difference of opinion amongst authorities. Local
GENERAL CONSIDERATIONS. 17
conditions affect the pavement in this regard to an im-
portant degree. The dampness of the climate, the
shade from buildings, the cleanliness of the streets,
and the prevalence of snow and ice in winter are all
Statistics upon the question of relative safety of
wood, asphalt, and granite have been collected by
Capt. Greene in this country and by Col. Haywood
in London, the attempt being made to determine
the number of miles travelled by horses upon each
kind of pavement to each accident due to slipperi-
The results of Col. Haywood seem to show that
of the three wood is the safest and granite the most
dangerous, while the results of Capt. Greene show
asphalt to be the best and wood the worst in this
Col. Haywood's observations were all taken on
London streets, and are as follows :
Miles travelled to each fall on
Granite. Asphalt. Wood.
In dry weather, 78 223 646
" damp " 168 125 193
" wet " 432 192 537
All observations, 132 191 330
The observations were made when dry weather
prevailed, and therefore are somewhat unfavorable to
granite, which is safest when wet.
Capt. Greene's observations were made in several
American cities, and showed the distance travelled to
each fall to be, on granite 413 miles, on asphalt 583
miles, and on wood 272 miles. The observati<
1 8 A TEXT-BOOK ON ROADS AND PAVEMENTS.
wood in this series were too few to give a reliable in-
dication, and it is to be observed with regard to all of
them that slipperiness is largely affected by the con-
dition in which the surface is maintained, and it is
therefore difficult to draw any general conclusions
which would fit all cases.
All hard pavements are slippery when muddy and
wet, and cleanliness is the necessary condition *of
Wood and asphalt, if clean, are least slippery when
dry and most so when simply damp. Granite, after
the surface becomes worn and polished, is most slip-
pery when dry and least so when wet.
Under a light fall of snow both wood and asphalt
become very slippery, and in freezing weather wood
sometimes becomes slippery through the freezing of
the moisture retained by it.
No statistics are available as to the safety of brick
pavements, but it is thought a desirable material in
It may also be remarked, that the danger of a horse
falling upon any pavement depends very largely upon
the training of the animal and whether he be accus-
tomed to the particular surface in question.
ART. 8. DURABILITY OF VARIOUS SURFACES.
The durability of a road or pavemjnt is dependent
upon so many circumstances connected with local con-
ditions, the nature of the traffic, methods of con-
struction, and efficiency of maintenance, that any
comparison of the various kinds of pavement in this
respect is difficult and likely to be misleading.
GENERAL CONSIDERATIONS. ig
The qualities which especially affect the durability
of the road may be partially enumerated as follows :
(1) The hardness and toughness of the material com-
posing the surface, upon which depends the resistance
of the surface to the abrading action of the wheels and
horses' feet passing over it.
(2) The firmness of the foundation, which serves to
distribute the loads over the road-bed, and keep the
(3) The drainage of the road-bed, which can only
properly sustain the loads which come upon it when it
(4) The permeability of the surface, which should
form a water-tight covering to serve the purpose of
keeping the foundation and road-bed in a dry con-
(5) The resistance of the materials of the pavement
to the disintegrating influences of the atmosphere, and
to the action of the weather.
The relative importance of these various factors, in
any particular case, depends largely upon the nature
and extent of the traffic which is to pass over the
The amount of traffic to which a street is subjected
is usually estimated in terms of tons per foot of width
of street, by observing the number of teams passing a
given point during certain times, classifying them, and
assigning an average value of load to each class. The
wear of the surface will naturally be somewhat propor-
tional to the amount of traffic. The life of a pave-
ment is, however, affected by other conditions, and
hence cannot always be inferred from the amount of
20 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Traffic may also be classified according to its nature
as heavy or light, depending upon the weight of indi-
vidual loads which are carried. It is the heavy loads
borne upon narrow wheel-tires that do the greatest
damage to a pavement, and hence the nature rather
than the amount of traffic determines the character of
Granite blocks, where a firm unyielding foundation
is employed, give the hardest and most durable surface
of any of the common pavements. This is especially
the case under very heavy loads.
Asphalt and brick rank next to stone, and when
well constructed are satisfactory under any but the
heaviest traffic. The relative durability under wear of
brick and asphalt is a matter of doubt, both materials
being subject to considerable variations in quality,
and showing varying results in different localities, due
both to differences in the quality of the material and
in the methods of construction.
Wood is less durable and only suitable for com-
paratively light traffic, unless its other advantages be
considered worth the high cost of maintenance under
heavier traffic, as has been the case in London, where
wood has been largely used under traffic which required
its renewal every four or five years.
Broken stone wears rapidly under moderately heavy
traffic, and should be employed only on suburban
streets or country roads used mainly for light driving
or a small amount of traffic.
The durability of any pavement also depends largely
upon the system employed for maintaining it, and
upon its being kept clean. Cleanliness is specially
important with wood, asphalt, and broken stone.
Brick or stone blocks are not so much injured by
The wear of a pavement also depends largely upon
the smoothness of the surface, as the impacts to which
the material is subjected are produced by irregularities.
So that the most durable material may not always give
the greatest resistance to wear.
DRAINAGE OF ROADS AND STREETS.
ART. 9. NECESSITY FOR DRAINAGE.
THE road-bed, usually formed of the natural earth
over which the road or pavement is to be constructed,
must always carry the loads which come upon the
road surface. Where an artificial road surface or
pavement is employed, the earth road-bed is protected
from the wear of the traffic, and the \vheel loads com-
ing upon the surface are distributed over a greater
area of the road-bed than if the loads come directly
upon the earth itself ; but the loads are transferred
through the pavement to the road-bed, and not sus-
tained by the pavement as a rigid structure.
The ability of earth to sustain a load depends in a
large measure upon the amount of moisture contained
by it. Most earths form a good firm foundation so
long as they are kept dry, but when wet they lose
their sustaining power, becoming soft and incoherent.
When softened by moisture the soil may be easily
displaced by the settling of the foundation of the
road, or forced upward into any interstices that may
exist in its superstructure.
In cold climates the drainage of a road is also im-
portant because of the danger of injury from freezing.
Frost has no disturbing effect upon dry material, and
DRAINAGE OF ROADS AND STREETS. 23
hence is an element of danger only in a road that re-
In order, therefore, that the loads may be uniformly
sustained, and the surface of the road kept firm and
even, it is evidently of first importance that the road-
bed be maintained in a dry condition. This may be
accomplished by the use of an impervious road cover-
ing, by proper underdrainage, or by a combination of
the two, as may be necessary in any particular case.
An impervious surface is always desirable, not only
as a means of keeping the road-bed dry, but also as a
protection to the pavement itself against the disin-
tegrating action of water and of the weather upon
the materials of the surface. Such a surface is not,
however, always practicable, and other means must
often be used to free the road from water.
The necessity for underdrainage in any case de-
pends upon local conditions, the nature of the soil, and
the tendency of the site to dampness, as well as the
permeability of the surface.
The object should be as far as possible to prevent
water from reaching the road-bed, and to provide
means for immediately removing such as does reach it
before the soil becomes saturated and softened.
ART. 10. SURFACE DRAINAGE.
The drainage of the surface of a road is provided for
by making the section higher in the middle than at the
sides, with ditches or gutters at the edges of the road
along which the water is conducted until it may be dis-
posed of through some side channel.
The slope necessary from the middle to the sides of
24 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the road to insure good drainage depends upon the
nature of the covering, being less as the road surface
is more smooth and less permeable to water. It varies
from about I in 20 or I in 30 for broken stone to I in
40 or i in 60 for various classes of pavement, and for
asphalt sometimes as low as I in 80.
The form of section used is commonly either a con-
vex curve, approximately circular, or it is made up of
two plane surfaces sloping uniformly from the middle
to the sides in each direction, and joined in the middle
by a small circular arc. There has been considerable
dispute among engineers as to which of these forms is
most desirable, although the general preference seems
to be given the plane section. It is not usually a
matter of special importance, provided the section
used is not too flat at the middle for good drainage,
and not too steep at the gutters for safety. In
places where surface-water must be carried for con-
siderable distances in gutters at the side of the road,
and provision must be made for a considerable flow,
the gutters may be deepened by increasing the slope
of the surface at the sides, or rounding off as much
as possible without making the slope too steep for
The road should also have a certain longitudinal
slope in order that the water may flow freely in the
gutters. This slope should be at least I in 200 in
most cases in paved streets, and somewhat greater
about i in 100 to I in 120 on broken-stone or earth
roads. Where longitudinal slopes are steep, some pro-
vision must be made to prevent the wash of the gut-
ters, and in such places it is specially desirable to take
the water from the gutters -as frequently as possible,
DRAINAGE OF ROADS AND STREETS. 2$
in order to make the gutter flow small. This may
often require, where no sewers exist, the laying of
a special pipe underground for the purpose.
On country roads the disposal of surface-water is
not usually a matter of difficulty, as it can be carried
along the road and run into the first convenient cross-
In towns the most satisfactory method of disposing
of surface drainage is through a system of storm
sewers, the water collected in the gutters being emptied
at frequent intervals into the sewers and thus quickly
removed from the surface of the street. In the ab-
sence of such a system it may often be necessary to
lay pipes, connecting with the nearest natural channel,
to relieve the gutters. In such cases catch-basins
should always be placed at the entrance to the pipe
to prevent it getting clogged by the dirt which may
be washed in from the gutter. On
lines of pipe of considerable length,
catch-basins should also be intro-
duced at intervals, to allow the
accumulated sediment to settle and
Fig. 3 represents a basin of this
kind. It may be formed for small
pipes, of a length of pipe set on
end with the lower end closed, or
where necessary a box built of
masonry may be employed.
In all cases it is important that the water which
falls upon the surface should be gotten rid of as soon
as possible, for so long as it remains upon the road it
it is an element of danger, both from its tendency to
26 A TEXT-BOOK ON ROADS AND PAVEMENTS.
wash the surface, and from its liability to penetrate
into the road and thus cause disintegration or settle-
ment. The best method of removing this water in
any particular case must be determined by a careful
study of local conditions, and its final disposal in the
case of the streets of a town is a special problem re-
quiring careful treatment.
ART. II. SUBDRAINAGE.
The drainage of the sub-soil of a road-bed may be
directed either to the removal from the road-bed of
water that percolates through the road covering, or to
the prevention of sub-surface waters from reaching
and saturating the road-bed.
The necessity for subdrainage, and the method to
be employed in any case, depends upon whether the
soil over which the road is being constructed is natu-
rally wet or dry, and whether the road-bed is so situ-
ated and formed as to give it natural drainage.
Where artificial subdrainage is necessary the drains
should be located, in so far as possible, with a view to
cutting off the supply of water before it reaches the
road-bed. To accomplish this to the best advantage
the local conditions must be observed, the sources of
this supply determined, and the nature of the under-
flow, if any exist, considered.
In many situations, particularly when the site of the
road is low and naturally damp in wet weather, it may
be advisable to place a longitudinal drain under each
side of the road. Such a construction is shown in
Fig. 4, which gives a section of a macadamized country
road with tile side-drains.
DRAINAGE OF ROADS AND STREETS. 2?
Frequently, as in many cases of a road along a side
slope, there is a well-defined flow of sub-surface water
from one side to the other, and in such case the water
may perhaps be intercepted by a single longitudinal
drain on the side of the roadway from which the
water comes. An example of this is shown in Fig. 5,
which represents a macadamized village street with
stone curb, gutters, and sidewalks.
In other cases, where the subsoil is of a very reten-
tive nature, or where the natural slope of the land is
in the direction of the length of the road, cross-drains
leading into a longitudinal side-drain or into side-
ditches may be expedient, and sometimes, especially
upon narrow country roads, a single longitudinal drain
under the middle of the road may give the best re-
sults, serving both to remove sub-surface water and
that which percolates through the road surface. Fig.
6 shows such a road, representing an ordinary earth
road with a tile centre-drain. Fig. 20 also represents
a stone centre-drain as sometimes used under a broken-
stone road over wet ground.
28 A TEXT-BOOK ON ROADS AND PAVEMENTS.
These longitudinal drains should be arranged to
empty as frequently as possible on country roads into
the natural drainage-channels, or in towns into sewers
arranged to convey the water rapidly away.
In general, systematic underdrainage will not be
necessary except in localities where the ground is natu-
rally damp from lack of natural drainage, or where an
underflow creates a tendency to wetness in the sub-
soil. In some localities, however, upon country roads,
where an impervious surface is not employed and the
soil is one that absorbs and retains water, it may be
necessary to provide subdrainage to remove water
that passes through the road surface. This is most
commonly done by a series of shallow cross-drains or
by a single longitudinal one in the middle of the road.
In a town where sewers traverse the streets subsoil
drainage is easily arranged for in connection with the
sewers. Frequently blind-drains or stone-drains are
constructed underneath or alongside the pipes. In
other cases good drainage is secured by drains under-
neath the curb or gutter, which are connected with the
ART. 12. KINDS OF SOIL.
The material of which a road-bed is composed is
important because it determines to a large extent
DRAINAGE OF ROADS AND STREETS. 2Q
whether artificial drainage is necessary, and also what
method should be adopted for securing drainage.
Soils differ in their power to resist the percolation
of water through them, in the rapidity and extent of
their absorption of water with which they come in con-
tact, in the extent to which moisture renders them soft
and unstable, and in their power of retaining moisture.
A light soil of a sandy nature usually presents little
difficulty in the matter of drainage, as, while it is easily
penetrated by water, it is not retentive of moisture,
which passes freely through it without saturating it
unless prevented from escaping.
If the natural drainage, therefore, have a fall away
from a road-bed formed of such material it will usually
need no artificial drainage, and where subdrains are
necessary they may be relied upon to draw the water
from the soil to a considerable distance each side of
A nearly pure sand is more firm and stable, under
loads, when quite damp than if dry, although a fine
sand saturated by water which is unable to escape
may become unstable and treacherous.
Clays usually offer considerable resistance to the
passing of water through them, and are very retentive
of moisture. As a rule, however, a clay soil does not
absorb water readily, and requires that water be held
for some time in contact with it in order that it may
become saturated, although when saturated it is the
most unstable of soils. A clay that when dry will
stand in a vertical wall and support a heavy weight
when wet may lose all coherence and become a fluid
mass. When water comes in contact with a bed of
such clay, the outside becomes saturated
3O A TEXT-BOOK ON ROADS AND PAVEMENTS.
fluid before the moisture penetrates into it sufficiently
to even moisten it a few inches from the surface.
A clay soil is, therefore, always difficult to drain by
removing the water after it has soaked in, or by per-
mitting it to pass through the road-bed to the subdrains
beneath. Drainage, in such cases, may often be so
arranged as to prevent water from standing against
the road and thus prevent it from becoming saturated.
As the clay is comparatively non-absorptive, the water
which may come upon its surface, if allowed to escape
at once, will not penetrate into it, and hence will not
A heavy silt formation is sometimes met with which
is even more difficult to drain than a true clay. It is
nearly as retentive of moisture as a clay, strongly re-
sisting the passage of water through it, but at the same
time absorbs water quite freely when in contact with it.
Between the extremes mentioned above there are
a great number of varieties of soil which possess to a
greater or less extent the characteristics of either or
both, and gradually merge the one into the other. In
applying a system of drainage in any case, careful
attention should always be given to the characteristics
of the soil, as determining very largely the treatment
to be used.
In pervious, sandy, or gravelly soil drains may often
be effective for a distance of 30 or 40 feet through the
soil, while with a less pervious retentive clay the drain
may not act effectively more than 8 or 10 feet on each
DRAINAGE OF ROADS AND STREETS. 31
ART. 13. TYPES OF DRAINS.
For the purpose of draining the subsoil of the road-
bed the drains used may be either open ditches at the
sides of the roads or porous covered drains.
Open ditches are sometimes used on country ro?ds.
They are usually placed at the extreme edges of the
road, and must be deep in order to be effective. Being
so far from the travelled portion of the road, they can
only act satisfactorily as subdrains where the soil is
pervious and easily drained.
In other cases, where side-ditches are employed,
covered cross-drains must be introduced to carry the
water from the middle of the road to the ditches. Fig.
7 shows a section of a country road drained by side-
ditches. Where such ditches are employed the slope
of the sides should be made as gradual as possible, at
least ij- or 2 horizontal to I vertical, in order to dimin-
ish the danger of the washing of the banks, as well as
the liability to overturning of a vehicle over the edge.
Covered underdrains are to be preferred to open
ones for this use, and are more commonly employed
where efficient subdrainage is attempted. These
drains must be so arranged that they may be read-
ily penetrated by the water without becoming clogged
by earth washing into them. The types of drains
most commonly employed for this purpose are known
as blind drains, box or stone drains, and tile-drains.
32 A TEXT-BOOK ON ROADS AtfD PAVEMENTS.
For short lengths, such as transverse drains intended
to take the water from the subsoil into the side ditches
or drains, blind drains may frequently be economically
employed. They consist simply (as shown in Fig. 8)
of ditches cut into the soil
and filled with rounded
stones 3 to 6 inches in di-
ameter. Angular stones are
not desirable for this pur-
pose, as the object is to
leave openings into which
the water may penetrate
without difficulty, and thus
be led away. The top of
the stones must be covered over in some way before
filling in earth above in order to prevent the earth from
washing down and choking the drain. This is some-
times accomplished by using smaller stones at the top,
covered by a layer of coarse gravel. A layer of straw
or brush, or of sod turned roots upward to retain the
earth until it becomes thoroughly compacted, is a
common and effective method of protecting these
Stone-drains are commonly employed where stone is
plenty and cheap. They usually consist of rectangular
or triangular boxes formed of flat stones or bricks at
the bottom of a trench, which is then filled as in a blind-
drain so as to give ready access for water.
Figs. 9 and 10 show sections of stone-drains as com-
monly constructed of rough field-stones. The form
given in Fig. 9 is commonly known as a box drain.
Tile-drains are probably in general the most con-
venient and efficient for subdrainage. They are made
DRAINAGE OF ROADS AND STREETS.
of round, or U-shaped, unglazed drain-tile, laid, as in the
last case, at the bottom of a ditch filled with round
Fig. 1 1 shows the section of a tile-drain as commonly
The tiles are usually set end to end in the trench,
being held in place by small stones braced under-
The joints are thus left open to permit of the free
entrance of water. Collars for the joints may be ob-
tained and are sometimes used ^^^^^^^^^^^^^
where thought necessary to keep
larger material from washing into
and obstructing the tile. These
collars are rings of pipe into which
the ends of two adjoining sections
of the tile may be fitted, and they
thus serve also to hold the tile in
The filling of the trench above
the tile, as in the other drains,
should be arranged with a view to maintaining a
34 A TEXT-BOOK ON ROADS AND PAVEMENTS.
porous structure through which water may easily
pass. Sometimes flat stones are laid over the tile
resting with one edge on the bottom of the trench
and meeting above at the middle so as to form an
additional protection to keep any earthy matter from
choking the entrance to the tiles.
All of these drains should be deep enough to escape
The mouth of a tile-drain should also be protected
in some manner, as the porous tile is apt to be broken
and destroyed by frost when saturated. In some cases
the tile-drain is made to discharge through a short
length of stone-drain, or through a section of salt-
glazed sewer-pipe, which will not be injured by freez-
It is desirable in all cases to protect the mouths of
underdrains with a netting of some kind to prevent
the entrance of vermin, which may clog the openings
with their nests.
The slope of a porous drain may vary from about I in
40 to I in 100. In case a steeper slope be necessary, a
foundation or paving should be placed in the bottom
of the trench, which is otherwise liable to be eroded by
the current that may be produced.
ART. 14. CULVERTS.
Culverts are commonly required in road construction
for carrying under the road the small streams which
may be crossed by the road, or sometimes for carrying
the water collected in the gutters or ditches on the
upper side of the road to the lower side.
The waterway provided by a culvert must, for
DRAINAGE OF ROADS AND STREETS. 35
safety, be sufficiently large to pass the maximum flow
of water that is likely to occur, while for economy it
must be made as small as may be without danger.
The maximum flow of a stream depends upon a
number of local conditions, most of which are very
difficult of accurate determination. These are : the
maximum rate of rainfall ; the area drained by the
stream and its position ; the character of the surface
drained ; and the nature of the channel.
The maximum rate of rainfall varies in different
localities, and differs in the same locality from year to
year. It is commonly taken at about an inch an hour.
This is sometimes exceeded for a very short time and
over a small area, but is usually a safe value for a
watershed of any considerable area.
The approximate area of the watershed drained by
a stream is readily found, and its form is also impor-
tant as determining the distance the water must flow in
reaching the culvert under consideration, and to some
extent regulating the rate at which the water falling
upon the area will reach the culvert.
The maximum flow of a stream is also affected by
the physical characteristics of the watershed. The
permeability of the surface largely determines what
portion of the rainfall shall reach the stream ; while the
slope of the surface, its evenness, and its vegetation
have an effect upon the quickness and rate with which
the rainfall is received by the stream.
The determination of the maximum flow to be ex-
pected in any case from an examination of the locality
is therefore possible only as a very rough approxima-
tion. A number of formulae have been proposed for
such estimation, the use of which foriifi^cageof an
36 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ordinary culvert simply amounts to estimating the
quantity of water which would fall on the watershed in
the heaviest probable rain, and judging as well as pos-
sible from local conditions how much of it may arrive
at one time at the culvert. In some cases where a
more accurate determination is desirable it may be
advisable to measure the flow of the stream at high
water, and form an idea from such measurement as to
what may be expected at a maximum stage.
The amount of water that will pass a culvert in a
given time depends upon the form of the section, the
smoothness of its interior surface, its slope, and the
head under which the water is forced through. A
well-constructed culvert may be considered in comput-
ing its capacity as a pipe flowing full. Other culverts
or bridges must be treated as open channels.
Prof. Talbot gives (Selected Papers C. E. Club, Univ.
of Illinois, 1887-8) a formula for the rough determina-
tion of area required for waterway, derived from
Area waterway in feet C V (drainage area in acres) 3 .
C is a coefficient depending upon local conditions.
For rolling agricultural country subject to floods at
time of melting snow, and with length of valley 3 or 4
times the width, C -J. When the valley is longer,
decrease C. If not affected by snow and with greater
lengths, C may be taken at \, \, or even less. For steep
side slopes C should be increased.
For most cases in practice the size of waterway
required may be determined from the knowledge which
usually exists in the vicinity regarding the character of
DRAINAGE OF ROADS AND STREETS. 37
a stream, from the sizes of other openings upon the
same stream, or from comparison with other streams
of like character and extent in the same locality.
Where data of this kind do not exist, careful exami-
nation of water-marks on rocks, the presence of drift,
etc., may be made to determine the height to which
water has previously risen.
For small flow of water box culverts of stone or pipe
culverts are commonly employed. Wooden box cul-
verts are also sometimes used, constructed of planks or
heavy timbers, but should be avoided in so far as pos-
sible on account of their perishable character, and con-
sequent lack of economy.
Pipe culverts are the most efficient in use, and as
they can now be constructed quite cheaply in most
parts of the country, are coming into very general use.
The efficiency of a pipe culvert may frequently be
greatly increased by arranging it to discharge under
a considerable head at times of unusual flood. This
requires that the water shall freely flow away below
the outlet, and that the surface of water above the
culvert may stand higher than the head of the pipe.
Pipe culverts may be constructed either of salt-
glazed vitrified sewer-pipe, or of iron water-pipe.
The iron pipe possesses greater strength, and is pref-
erable where a firm foundation is not easily obtained,
as it is not so easily broken by any slight settlement.
In laying pipe culverts, they should be placed on a
solid bed, and the earth be well tamped about them.
It is desirable to have the bottom of the trench exca-
vated to fit the lower part of the pipe, depressions
being formed for the sockets. It is necessary in every
case that the pipe be firmly and uniformly supported
38 A TEXT-BOOK ON ROADS AND PAVEMENTS.
from below, in order that the culvert may not be
broken by settlement, which is especially likely to
occur in new work.
It is desirable that the joints in the pipe be made
water-tight, especially where the culvert is likely to flow
full or under pressure, as any water escaping through
the joints will tend to cause a wash beneath the pipe
and undermine the culvert. Joints are commonly
filled with clay. Where strength is needed the use of
hydraulic cement mortar is preferable, and sometimes
the small end of the pipe is roughened on the outside
and the socket on the inside in order that the cement
may hold more firmly.
Care should be taken that the culvert have sufficient
slope and be so placed that water may not stand in
it, in order to prevent injury from freezing, and the
top of the culvert pipe should be at least two feet
below the road surface to avoid crushing.
The ends of pipe culverts should be set in masonry
walls to give protection against the washing of the
face of the embankment, hold the ends firmly in place,
and prevent the entrance of water into the earth on
the outside of the pipe.
These walls to give efficient protection must be of
substantial construction, going down to a solid founda-
tion below the bed of the stream. They may be built
of rubble masonry, and should be laid up in hydraulic
cement mortar. Such construction is represented in
Fig. 12. The wall must extend far enough on the side
to sustain the earth of the embankment from the
waterway, or wing walls may be used extending up
stream for this purpose. The waterway should be
DRAINAGE OF ROADS AND STREETS.
paved above the culvert far enough to prevent scour-
ing at the base of the wall.
For quite small streams the walls may sometimes
be omitted if the face of the embankment about the
entrance to the pipe and the waterway for some dis-
tance above and below be riprapped. Where it is
necessary to economize in the cost of construction,
this method is preferable to the use of very light end
On streams too large for a single pipe it is often
economical to lay two or three pipes side by side,
rather than to construct an arch or the open way of a
bridge. In laying large pipes it is usually advisable
to place a broken-stone or concrete foundation under
the pipes throughout their lengths to insure uniform
Stone Culverts. Culverts of stone may be either
arch culverts or box culverts. Box culverts are usually
formed of two side walls and a cover. The side walls
consist usually of rubble stonework laid up dry or in
mortar, as the case may be. Where the stream to be
40 A TEXT-BOOK ON ROADS AND PAVEMENTS.
carried is of small importance, and the capacity of the
culvert not greatly taxed, dry walls may give satis-
factory results, but when the culvert is likely to flow
full at certain times it should be laid up in hydraulic
cement mortar, and in any case the greater stability
given by the mortar would be well worth the small
additional cost. Fig. 13 shows a section of the ordi-
nary form of box culvert. The use of head walls and
paving the waterway for a short distance is necessary
for these as for pipe culverts.
Where suitable stone is available, box culverts are
easily constructed and economical. They are com-
monly used for openings 2 to 4 feet in width and
2 to 5 feet in height. The width that may be used
depends upon the available cover stones. Where the
allowable width is not sufficient to give the needed
area of waterway, a double culvert may sometimes be
used to advantage. This consists of two openings
with a middle wall to support the covers.
The culvert's opening should always be large enough
to admit of a man passing through it for the purpose
DRAINAGE OF ROADS AND STREETS. 4!
of cleaning it at least 1 8 by 24 inches. The side walls
should extend downward below the bottom of the
culvert sufficiently to obtain a good foundation, and
the thickness required for the side walls usually varies
from one half to three fourths the height, depending
upon the pressure likely to come against them.
In many cases for small work the side walls, instead
of extending downward, rest upon the paving which is
extended under them. This gives a somewhat less
expensive construction, and is often satisfactory on
The cover stones may be from -J- to the span in
thickness, and should be long enough to have a bear-
ing upon each side wall of at least one half the thick-
ness of the wall.
Arch culverts are used for openings too large to be
made of the box form or of pipes. The discussion of
arches and also that of bridges in general is outside
the proper scope of this book. For ordinary country
bridges wooden trusses are most commonly employed,
on account of their comparative cheapness. For short
span bridges a satisfactory and economical construc-
tion, which has recently come extensively into use,
consists in placing a number of wrought-iron eyebars
across the opening from abutment to abutment at
short distances apart. Brick arches are then used to
span the spaces between the eyebars, which are tied
together with wrought-iron rods, and the roadway is
then constructed over the bridge in the same manner
as upon the earth road-bed.
Concrete culverts may sometimes be used to ad-
vantage where they can be cheaply constructed. They
are usually made in oval form, the bottom being first
formed by ramming the concrete upon the foundation
42 A TEXT-BOOK ON ROADS AND PAVEMENTS.
so as to form a curved channel 4 to 6 inches thick.
The upper part is then constructed as an arch upon a
centre, which is left until the mortar has set. For the
small openings for which these culvert sare employed
the thickness of concrete in the arch may be from
to of the width of opening. The concrete for such
work should be made of hydraulic cement, in the
manner employed in constructing the foundations for
pavements. (See Art. 47.)
Abutments for small bridges should be laid upon
solid foundations, and built of hydraulic cement
mortar, the back of the abutment wall being made
impervious by coating it with mortar. A common and
safe thickness of abutment is to make the thickness
^Q of the height. The waterway between abutments
should be paved to prevent scouring out the founda-
ART. 15. WATER-BREAKS.
Upon heavy gradients on country roads, continuous
for any considerable distance, water-breaks are com-
monly placed at frequent intervals to collect the water
which flows down the surface of the road and turn it
into the gutters or side-ditches. They should only be
used on grade steep enough to make them essential, as
otherwise they form an obstruction to traffic. They
consist of broad shallow ditches, and should be arranged
to carry the water from the middle of the road each
way to the gutter, thus forming a V with the vertex
uphill and at the middle of the road. This arrange-
ment permits teams following the middle of the road
to cross the ditch squarely. It is desirable that these
cross gutters be paved to prevent washing during
LOCATION OF COUNTRY ROADS.
ART. 1 6. CONSIDERATIONS GOVERNING LOCATION.
THE determination of a line for a proposed road
involves the examination of the country through which
the road is to pass with reference to its topographical
features, the nature and extent of the traffic that it
may develop, and the local interests that may be
affected by the position of the road.
The simplest form that this problem can take is that
in which two points, as two towns, are to be connected
by a road for the purpose of providing for a traffic
between them, the nature and amount of which is
approximately known. In this case it is only neces-
sary to examine the topography of the intervening
country and select the line over which, taking into
account the costs of construction and maintenance,
the given traffic may be most economically carried.
In most cases in practice, however, the problem
does not have this simple character, and in fact loca-
tion can seldom be determined by considerations of
economy alone. The position of the line will be modi-
fied by local needs, such as the necessity of providing
for the traffic of villages or farms intermediate between
the ends of the road, which may often cause deviations
44 A TEXT-BOOK ON ROADS AND PAVEMENTS.
from what would be the best line if the interests of
the terminal points alone were considered.
Questions of the desirability of various lines for the
comfort and convenience of travel, and the pleasure to
be derived from the use of the road, dependent upon
aesthetic considerations, may also frequently operate
to change the line from what would seem proper from
a strictly economic point of view.
In thickly settled communities, as in most parts of
the United States, the roads are in the main already
located, the necessity for the location of new ones
does not often arise, and when it does occur is usually
mainly determined by the local needs and requirements
The economic considerations involved in the location
of roads are of two kinds : those relating to the accom-
modation of traffic, and those relating to its economic
conduct. The first deals with the necessity of the road
to the community, the second with the cost of operat-
ing it. The first involves the general question of the
advisability of any road, and how it can be placed to
give the greatest freedom to the movement of travel.
The question is as to the value of the road to the gen-
eral community and its location to secure the greatest
good for the least outlay, without taking into account
the details of location which may affect the cost of
transportation. The value of the road as developing
trade in a town or bringing a farm nearer to market
would enter into consideration. The accommodation
of traffic requires that a road be located with a view to
the convenience of its use by the largest portion of the
traffic, as well as with a view of developing traffic.
The position of a road that will best accommodate
LOCATION OF COUNTRY ROADS. 45
traffic is that in whichj other things being equal, the
mass of traffic need be moved the least distance in
reaching its destination ; or, in other words, that for
which if each ton of freight be multiplied by the dis-
tance through which it must be moved the summation
of the resulting products will be a minimum. If there
be differences in the nature of the routes over which
the road may be constructed, they may be considered
as equivalent to changes in the relative effective lengths
of line for purposes of comparison.
The ordinary problem of location deals mainly with
considerations of the second class. It consists for the
most part in the relocation of portions of old roads,
of making such changes in position when improving a
road as may tend to reduce the cost of conducting
traffic over it, and render it more convenient and
pleasant for the use of travel, or of determining the
details of alignment and grade upon a new road which
is approximately fixed in position by the purpose of
The most economical location is that for which the
sum of the annual costs of transportation, the annual
costs for maintenance, and the interest on the cost of
construction is a minimum.
The cost of conducting transportation is affected by
the rate of grade of the road, the amount of rise and
fall in it, and the length of the road. The rate of
grade is important, because it limits the loads that can
be hauled over the road, or determines the number of
loads that must be made to transport a given weight
of freight, as well as fixes a limit to the speed of travel.
The amount of rise and fall affects the expenditure of
power required to haul a load over the road. The
46 A TEXT-BOOK ON ROADS AND PAVEMENTS.
length of the road has an effect upon the amount of
work necessary to haul a load over it, the time required
for a trip, and the cost of maintaining the road surface ;
each of which, other conditions being the same, is
directly proportional to the length.
The cost of construction depends upon the accuracy
with which the line of the road is fitted to the surface
of the ground, as determining the amount of earth-
work and cost of bridges and culverts ; upon the
character of the ground over which the road is to be
built, which affects the cost of executing the work and
determines the necessity for and expense of drainage ;
and upon the cost of land for right of way. All of these
items must be considered in any comparison of the
cost of constructing on various routes. Special care
should be taken in selecting a line to avoid bad ground,
such as swamps, upon which construction may be diffi-
cult and expensive. The availability near the line of
the road of materials needed for surfacing may also
become a matter of importance in the cost of construc-
tion, and have an influence in determining location.
The relative importance of the various elements af-
fecting the choice of a line depends upon the nature
and amount of the traffic to be provided for, and upon
the character of the road surface to be used. Where
the traffic is heavy, the importance of reducing the
cost of moving it by lessening grades and distance will
be greater than where the traffic is light, and the cost
of construction may be correspondingly increased for
that purpose. If a smooth surface be employed, upon
which traction is light, the value of reducing grades
will be greater and the value of reducing distance less
than with a surface of poorer tractive qualities.
LOCATION OF COUNTRY ROADS. 47
ART. 17. LENGTH OF ROAD.
Changes in the length of a road affect all portions of
the traffic in the same manner, and the expenditure of
power and loss or gain in time occasioned by them are
in general directly proportional to their amounts.
The value of any considerable saving in length may
usually be considered as equal to the same percentage
of the whole cost of conducting the traffic that the
saving in distance is of the whole length. If, therefore,
a rough estimate may be made of the annual traffic to
be expected upon a given line of road and of the cost
of carrying the traffic, this cost divided by the length
in miles through which the traffic is moved will give
+he annual interest upon the sum that may reasonably
be expended in shortening the road one mile, or upon
the value of a saving of a mile of distance; or di-
viding by the number of feet of distance will give the
value of saving one foot.
It is to be noted, however, that the cost of the work
of transportation is not necessarily proportional to the
amount of w r ork done, and consequently this method
would not be strictly accurate even were the data as to
traffic and costs readily obtainable. An estimate of
this character at best amounts to only a rough guess,
but it may often be of use as an aid to the judgment
in deciding upon the value of a proposed improvement
involving a considerable change of length in a road.
Where the road is so situated and the saving in
distance proposed is such that it would enable teams
to make an additional trip per day in the hauling of
freight, the difference in cost of transportation is quite
tangible and readily estimated ; but where
48 A TEXT-BOOK ON ROADS AND PAVEMENTS.
of a more indefinite nature, or the saving proposed
insufficient to admit of additional trips, the value of
the difference of length depends upon the value to
other work of the small portions of time of men and
teams which may be saved by the shorter route a
value which exists, but is difficult to estimate.
There is also a value in the saving of distance due
to the advantage to the community of bringing the
various points closer together, such as bringing two
towns into closer relations or bringing country property
nearer to markets. The method of considering the
cost as proportional to the work done will therefore
probably give a fair idea of the actual economy in any
saving in the work of transportation.
The value of reducing distance varies with the
character of the road surface. As the cost of transpor-
tation is less over a smooth than over a rough surface,
on account of the lighter traction, the value of reduc-
ing distance is also less on the smooth surface.
The value of saving distance also is greater on a
road where the ruling gradients are steep than upon
one with light gradients, because of the greater num-
ber of loads necessary to move the same traffic.
The cost of maintenance of a road varies with its
length, and under similar conditions may be con-
sidered, like the costs of transportation, to be directly
proportional to the length of road.
The saving in cost of maintenance from decreasing
distance must of course be added to that in cost of
transportation in order to find the actual value of a
change of length.
The value of straightness for a country road is fre-
quently very much overrated. Considerable devia-
LOCATION OF COUNTRY ROADS. 49
tions from the straight line may often be made with
but slight increase in length, and there seems to be no
good reason for insisting upon absolute straightness.
The error is commonly made of sacrificing grade and
expense in construction to the idea of straightness
without the attainment of any considerable saving in
It involves in many cases the injury of the beauty of
the road and of the landscape, with no compensating
ART. 1 8. RISE AND FALL.
By the amount of rise and fall is meant the total
vertical height through which a load must be lifted in
passing in each direction over the road. It is distinct
from and independent of the rate of gradient.
The minimum amount of rise and fall is found
where the rise is all in one direction and the fall in
the other, each being equal to the difference of eleva-
tion of the terminal points. Any increase in the rise
and fall beyond this amount is represented by the rise
encountered in passing from the higher to the lower
terminus. It affects the traffic equally in each direc-
tion, and requires a certain expenditure of power to
lift the traffic through the given rise in each direction.
If the cost of developing the work necessary to
overcome rise and fall be the same as that of develop-
ing an equal amount of work to overcome distance, the
rise and fall may be evaluated in terms of distance,
and any change in rise and fall may be considered as
though it were a difference in distance and treated as
in Art. 17.
50 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Where the rate of grade is less than the angle of
repose of the wheels upon the road surface (see Art.
2) the work necessary to overcome rise and fall will
be that which will lift the load through a vertical height
equal to the amount of rise to be considered. When
the rate of grade is greater than the angle of repose, an
additional amount of work must be done in applying a
resistance to prevent the too rapid descent of the vehicle
in going down the grade. The amount of this work in
any case equals the work done in lifting the load to a
height equal to the difference between the actual rise
of the grade in question and the rise of a grade of the
same length and a rate equal to the angle of repose.
Thus on an ordinary earth road whose resistance to trac-
tion where level is 100 pounds per ton, suppose a grade
to occur of 8 feet per 100, 1000 feet in length. For
the road surface we have 100 -f- 2000 == .05, and the
angle of repose is a 5$ grade. Then 8$ 5$ = 3$, or
the brake-power necessary to secure uniform motion,
is the same as would be necessary to haul the load
up a 3$ grade, and a grade of 3 in 100 for 1000 feet
gives 30 feet. The work to be done in holding back
the load for the looo-ft. grade is therefore the same as
would lift the load through a vertical height of 30 feet,
or the fall of 8 feet per 100 for 1000 feet has the same
effect as 30 feet of rise in the same direction, pro-
vided brake-power costs the same as animal power.
The value of rise and fall in terms of distance will de-
pend upon the nature of the road surface, as the work
necessary to lift a given load to a given height is a con-
stant, while the work done in hauling a load over a given
distance will vary with the resistance offered to traction
by the surface. Thus, taking the surface as above, the
LOCATION OF COUNTRY ROADS. 51
work of lifting one ton through a rise of I foot is 2000
foot-pounds, while with a tractive force of 100 pounds
per ton 2000 -=- 100 = 20 feet, the distance a ton may
be moved on the level surface in developing 2000 foot-
pounds of work. Therefore I foot of rise or fall may
be considered as equivalent to 20 feet of level distance,
and the value of reducing the amount of rise and fall
may be found from that for reducing distance.
If the road considered were a first-class Macadam
road, with resistance of 40 pounds per ton, I foot of
rise or fall would equal 2000 -f- 40 = 50 feet of distance.
ART. 19. RATE OF GRADE.
The effect of any change in the ruling gradient upon
a road depends to a considerable extent upon what
portion of the traffic may be carried in full loads. The
lighter portions of the traffic are not so seriously
affected by heavy gradients as the heavy portions,
although there is an advantage in light gradients for
any driving. The rate of speed which may be em-
ployed will be less upon the portions of the road having
heavy grade, and the time occupied in a trip over the
road is therefore affected somewhat by the rate of
The desirability of a road for general driving is
also much influenced by the gradients employed, as is
that value of the road which has for a basis the effect
it may exert upon the attractiveness of the locality.
These things all have a certain financial value, which
of course it is quite impossible to estimate with any
degree of accuracy, but which should be considered in
determining the allowable maximum gradient in any
case in practice.
52 A TEXT-BOOK ON ROADS AND PAVEMENTS.
For heavy traffic, such as the transfer of goods from
one town to another or the marketing of country prod-
uce, the limitation of load placed upon the traffic by
the gradient is a matter of importance, the effect of
which is calculable upon the cost of transportation.
If in any case the approximate amount of this heavy
traffic which is likely to be carried in full loads be de-
termined, the relative costs of its transportation over
two lines of differing gradient, other conditions being
similar, will be nearly proportional to the number of
loads required to move the traffic over each gradient.
In estimating the value of reducing the rate of grade,
it may be considered, as in the case of a reduction of
length, that its value to the community is represented
by the saving in annual costs of transportation, and
that the amount that may reasonably be expended in
increased cost of construction to effect a reduction of
gradient is the sum upon which this annual saving is
The length of a road and the amount of rise and
fall on it determine the amount of work that must be
done in hauling a load over the road. The rate of
gradient, on the contrary, does not affect the amount
of work necessary to move the traffic, but it limits the
work that a horse may do at one trip.
The establishment of a proper rate for the ruling
grade of the line is, therefore, usually the most im-
portant point in location. In localities where light
gradients are easily obtained the problem of location
is greatly simplified.
By referring to Article 3 the comparative loads that
a horse may draw up different grades will give some
idea of the importance of carefully considering the
LOCATION OF COUNTRY ROADS. 53
question of gradient. In nearly all cases in practice
there is a considerable latitude within which gradients
may be chosen. It is usually a question of heavier
gradients as against greater distance and larger first cost
for the road. It may be remarked that it is only under
exceptional circumstances that it is either necessary
or advisable to use a steeper gradient than 5# on
the new location of a country road of any importance.
Grades steeper than the ruling gradient may some-
times be introduced over short distances without
impairing the efficiency of the road, as horses are usually
able to exert for a short time a force much greater than
they can continuously exert. If the length of grade be
quite short, 200 or 300 feet, a horse can about double
his ordinary power in passing it.
Where long steep grades must be used, it is desirable
to break them by short stretches of lighter gradients to
provide resting-places for horses.
Heavy gradients also have the disadvantage of retard-
ing traffic in the direction of falling grade, and, as
suggested in Art. 18, of requiring the expenditure of
work to hold the load from too rapid descent.
ART. 20. EXAMINATION OF COUNTRY.
For the purpose of obtaining the requisite data
upon which to base the location of a road, it is neces-
sary that a careful examination be made of the topo-
graphical features of the country through which the
line is to pass. The relative elevations of the termini
of the line and of intermediate points should be
obtained, and the directions and steepnesses of the
various natural slopes determined.
54 A TEXT-BOOK ON ROADS AND PAVEMENTS.
If a line were to be located connecting points at long
distances from each other, as sometimes occurs in
railway location, it would be necessary to study the
general configuration of the country, noticing the di-
rection of flow of the streams, and the location and
elevations of the various passes in the ridges through
which it might be possible to carry the line. Usually,
it would be found that the country is composed of a
series of valleys, separated by ridges, branching in a
systematic manner from the main watercourse of
the region, and that the passes in the ridges occur at
the head of side streams, and especially where streams
flowing into valleys on opposite sides of the ridge have
their sources near each other.
In the location of common roads, however, the prob-
lem is ordinarily of a less extended nature, and may
consist in joining two points lying in the same valley,
or in joining points in adjacent valleys by a line pass-
ing over a ridge. In these cases it is only necessary to
take into account the slope of the valleys in question,
the positions and elevations of available passes, and the
side slope of the ridges.
The slope of the bed of a valley, in hilly country,
usually forms a concave curve, the rate of slope gradu-
ally increasing from the lower to the upper end. In a
valley of considerable length this increase in the rate
of slope may be very gradual or, in short valleys rising
to a considerable height, it may be more sudden. The
profile ABCD in Fig. 14 shows the slope of a short
valley which decreases in slope from about ten feet
per hundred at the upper end to about two feet per
hundred at the lower end.
When a map of the country to be traversed is avail-
LOCATION OF COUNTRY ROADS. 55
able, showing the positions and elevations of the points
controlling the location, the work is very much simpli-
fied, the reconnaissance may for the most part be
limited to a study of the map, and the routes may be
sketched upon the map to be tried in the field. If the
map at hand is an accurate contour map on a sufficiently
large scale, the entire location may be worked out in
detail upon the map, leaving only the work of staking
out the line to be done upon the ground.
Maps may be obtained, in most parts of this country,
upon which the horizontal positions of points may be
readily fixed with sufficient accuracy for the purposes
of the preliminary examination. Where such maps are
not obtainable, the positions of points must be ascer-
tained and a rough map prepared. For this purpose
directions may be measured with a pocket compass,
and distances estimated or obtained by the use of an
odometer or pedometer, as may be most convenient.
Differences of elevation are easily obtained with a
fair degree of accuracy by the use of an aneroid ba-
rometer, and slopes may be measured with a hand
Where the rough means ordinarily employed in the
reconnaissance are not sufficiently accurate to deter-
mine the controlling points of the lines to be adopted,
a more complete examination of the country may often
be made by a rapid topographical survey by means of
the transit and stadia method.
Whatever means may be adopted for doing the
work, the preliminary examination should determine a
map showing the approximate positions of the con-
trolling points through which the road must pass, and
56 A TEXT-BOOK ON ROADS AND PAVEMENTS.
enable a rough sketch to be made of the slopes of the
country through which the line is to be run.
ART. 21. PLACING THE LINE.
After the preliminary examination of the locality is
complete and the positions and elevations of the con-
trolling points of the line are known with reference to
each other, the line must be selected and run in upon
the ground, or, if the reconnaissance is not conclusive
as to the position of the best line, it is advisable to
run in two or more lines and make a more detailed
comparison between them.
The controlling points of a line are those points at
which the position of the road is restricted within
narrow limits, and is not subject to change. These
may be points where the location is governed by the
necessity of providing for traffic, or points where the
position of the line is restricted by topographical con-
siderations, such as a summit over which the line is to
pass a ridge or a favorable location for a bridge.
Where the line is to be located to an uniform gradi-
ent, it should be started from the controlling point at
the end of the grade, which is usually the summit. It
is then laid off along the slope in such manner as to
cause it to have continuously the rate of grade decided
upon. Taking D (Fig. 14) at the summit of the valley
as the controlling point, it is seen that the distance
from C to D is sufficient to give a gradient of 10 in
100 by following directly down the valley, and the line
with that gradient may be run in that manner.
The maximum gradient from A to C is, however,
only 5 in 100, and if thought advisable the same maxi-
58 A TEXT-BOOK ON ROADS AND PAVEMENTS.
mum gradient may be used between C and D by run-
ning the line DHC diagonally down the slope, as
shown. This line, having one half the gradient, will
have about twice the length of the line CD.
In running this line it is started from the highest
point of maximum grade, and points at the surface of
the ground are continually selected, in advance of the
placing of the line, which are at the proper elevation
to permit the grade to pass through them. This may
be accomplished by setting off the angle of the gradi-
ent upon the vertical circle of the transit, or upon a
gradienter, and sighting upon a rod which is moved
until the line of sight strikes it at the same height from
the ground that the instrument is setting. The points
for the line may also be found by running a line of
levels ahead of the transit line (a hand level is conveni-
ent for this purpose) and pacing distances upon which
to reckon the gradient, the distances and elevations
being frequently checked upon those of the measured
The location of a gradient upon a common road
differs from that upon a railroad only in that steeper
gradients are used, sharper curves or angles may
be employed, and the gradients need not be lessened
on ordinary bends or curves. If the line is to make
a turn upon the slope as at H, the grade should be
flattened at the turn, and a curve of as large radius as
possible, without too great expense for grading, be in-
In a manner similar to the above a line might be
run from D on the other side of the valley, which
using a 5$ gradient would give the line DML, reach-
ing the bed of the valley at the point L< A lighter
LOCATION OF COUNTRY ROADS. 59
gradient may be obtained from A to D by starting
from D and going down by a continuous gradient of
4 in 100 on the line D F G A, and greater or less rates
of descent may be adopted and lines corresponding to
them located, as may be considered advisable.
The centre-line for a final location should be care-
fully run, and points permanently marked from which
it may be relocated when necessary. An accurate line
of levels should also be run over the centre-line and a
profile drawn, upon which the grades may be estab-
lished and earthwork estimated.
After placing the centre-line, topography should be
taken carefully upon each side of the line for some
distance, and a map drawn showing the topography
and giving elevations by means of contours. This will
serve to show whether the line is placed to the best
advantage, and whether any changes are desirable.
This is especially necessary over rough ground or
where the line is on maximum gradient, as frequently,
and perhaps usually, the first line run will be useful
only as a preliminary line, which with its accompany-
ing topography will permit a proper location to be
ART. 22. COMPARISON OF ROUTES.
In selecting a line for the construction of a road the
principles already mentioned in the early part of this
chapter should be had in mind. The line must be well
designed to accommodate the traffic. It should have
as easy grades, short length, and small rise and fall as
is consistent with a reasonable cost of construction, in
order to give light costs for transportation and for
60 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Suppose in the case shown in Fig. 14 that it is desired
to connect the village at the point A with the point D
and with the roads leading through the passes at F
and /. Which line it will be the most advantageous
to adopt depends upon the relative importance of the
traffic to the various points considered.
The shortest, and probably cheapest, line from A
to D would be obtained by following the valley over
the line ABCD, which line, as shown by the profile,
would give a maximum gradient of 10 in ico between
C and D. The line FB joining the first line at B
would afford communication with the summit at F
with a maximum gradient of 5 in 100. If the traffic to
the point D be small and unimportant, so that addi-
tional expense in reducing the gradient from C to D
is unadvisable, these lines might prove a satisfactory
If, however, D be a point of importance and the
traffic from A toD heavy, it will be necessary to adopt
some means to reduce the gradient from C to D.
Leaving out of consideration the point F and consider-
ing B and C as points of minor importance, it might
be advisable to use the line ALMD with an uniform
5$ gradient from D to L, and branches to connect
with C and B. This would give a line but little longer
than the valley line, with only one half the ruling gra-
dient of that line.
If C is not important and can be neglected while B
and F must be considered, the line ABEHD has a
maximum gradient of 5 in 100, and connects A with
the points BF and D with a minimum total length of
road (being less than the valley line first considered).
When B and C must both be considered as of im-
LOCATION OF COUNTRY ROADS. 6 1
portance as well as F and D, the lines ABCHE and
HD will give a ruling gradient of 5 in 100 to both F
and D, and passing through B and C with a somewhat
longer line than in the last case.
This arrangement would make the length of haul
from A to D and F, each longer than by the first line
considered ; but the gradient to D would be lighter, and
the total length of road to be constructed and main-
tained would be less.
In case the points B and C are both unimportant,
and the line through the valley may be neglected, the
line AGFD provides a ruling gradient of 4 in 100 from
A to both F and D y and connects them with each
other, with about the same length as the shortest $$
gradient. When the point / must be taken into ac-
count, this line may be connected with / by the line
GI having a gradient of 4 in 100. This would give the
shortest line of uniform gradient to connect A with the
three points / F and D, and possibly a desirable line to
construct when the line through the point 7 is impor-
tant, even if the valley road from A to B is also neces-
The lines upon the side slopes are usually more ex-
pensive to construct than the valley lines, and the dif-
ferences of first cost of the various lines must of course
be considered. The importance of a difference in ex-
pense of construction depends upon the traffic to be
hauled over the road and the kind of surface to be
used. Where a broken-stone or gravel road is to be
constructed at considerable expense, the difference of
cost due to a change of location is relatively less im-
portant as being a less percentage of the whole cost,
while the difference of tractive effort due to grade is
62 A TEXT-BOOK ON ROADS AND PAVEMENTS.
of more importance, as being a higher percentage of
that upon the level, than would be the case with an
ordinary earth road.
As is easily seen from the above the choice of a
location for a road, while depending upon principles
easily stated, is in reality a matter requiring the use of
judgment, and is not readily reducible to a financial
comparison stated in money values, because the data
concerning the volume of the traffic and the cost of
conducting it can be determined only very roughly,
and contains many elements of error. For purposes of
comparison to aid the judgment, approximate data
may often be assumed or determined by a study of the
localities affected. In some cases observations may be
made of the number of teams of different classes pass-
ing certain points within certain times, to give a basis
for estimation of the annual volume of traffic. In
other cases, the annual hauling traffic, which is usually
the most important portion of the traffic in considering
location, may be estimated from the known interests of
the locality. Thus, if the produce of a certain section
of farming country must be hauled over a given road
to market, the amount of this produce may be esti-
mated from the acreage, and the relative number of
loads upon different grades then determined. The
cost per load over the road would then need to be as-
sumed in order to find the annual value of a reduction
In the same manner, the effect of changes of length
and in the amount of rise and fall may be found as
indicated in Arts. 17 and 18.
All of these items must be combined to find the rela-
tive total costs of transportation for each route. The
LOCATION OF COUNTRY ROADS. 63
cost of construction and of maintenance for each line
must then be estimated, and that line is the most ad-
vantageous which makes the sum of the annual charges
and the interest on the first cost a minimum. Where
several lines of traffic are to be considered together as
in Fig. 14, the cost of conducting all of the traffic by
each system of lines that may be employed must be
considered, the entire cost being made a minimum for
the system to be adopted.
ART. 23. CHANGING EXISTING LOCATIONS.
The problem that arises oftener than any other in
country-road location is that of improving short
stretches of road, where, owing to defective location,
the grades are unnecessarily heavy, the length unneces-
sarily great, or the ground over which the road may
pass such as to make its maintenance in good con-
dition difficult and expensive. The first of these is
the most common defect of ordinary country roads, as
shortness of distance has very commonly been obtained
by the disregard of the desirability of light gradients,
which in very many cases are easily obtainable.
The principles to be observed and methods of pro-
ce,dure in making the new location are exactly the
same as in an original location, save that in this case a
road already exists, and the question of economy is one
of determining whether the advantages to be obtained
in lessened cost and transportation and maintenance is
sufficient to warrant the expense of attaining new
right of way and constructing new road.
In Fig. 15 is given an example that is frequently met
in practice, where the existing road abed runs oye^CfH^T
64 A TEXT-BOOK ON ROADS AND PAVEMENTS.
point of a hill, with heavy gradient, while a line of very
much lighter gradient might be located around the
base of the hill through the pass at e, giving a greater
length of road, but much less rise and fall. The line
bed in the figure has a length about 800 feet greater, a
rise and fall 70 feet less, and a maximum gradient one
half as steep as the line bed. These relations are
shown in the profile in Fig. 15.
If the road in question be a common earth road,
i foot of rise and fall may be taken as equivalent, in the
work required to haul a load over it, to 20 feet of dis-
tance, and the 70 feet saved by the new location would
be equivalent to 1400 feet of distance. Hence, the
line bed may be considered as having an equivalent
length for purposes of traffic 1400 800 = 600 feet
shorter than the line bed. In addition to this, loads
may be taken over the new line in direction b to d
more than double, and in direction from d to b triple,
in weight those that can be taken by the same power
over the old line.
A further improvement of the line may also be
possible, if the new line can join the old one at a point
lower down than $, by running a lighter gradient than 5
in 100 from the point e. Thus the line efa would give
an uniform gradient of 4$, but would require the con-
struction of more new line.
In considering changes of location, it is also neces-
sary to take into account the interests of adjoining
owners. Houses and buildings are largely located with
reference to the existing position of the roads, and
changes in the position of a road may involve injury to
such property. The question then becomes largely
one of sacrificing the interests of the users of the road,
66 A TEXT-BOOK ON ROADS AND PAVEMENTS.
or those of the adjoining owners a question that
should be, but commonly is not, decided by consider-
ing what will be of most advantage to the general com-
IMPROVEMENT OF COUNTRY ROADS.
ART. 24. NATURE OF IMPROVEMENTS.
ORDINARY country roads may be classified as earth
roads, gravel roads, and broken-stone roads. The
larger number of common roads throughout this coun-
try belong of necessity to the first class. In a few of
the more enterprising communities the more important
roads are constructed of gravel or broken stone.
The percentage of roads of the better class is, how-
ever, very small and although there has recently been
a distinct improvement in this particular, the inability
of rural communities to at once raise the funds neces-
sary for the general construction of first-class new
roads will cause their increase to be very gradual.
Improvement in country roads may be of several
(1) Changes in location, by which better alignment
or better gradients may be obtained, or by which the
natural conditions of surface or drainage may be im-
proved. This has been discussed in Chapter III.
(2) Reconstruction of the road-bed, as in regrading
steep slopes to give lighter gradients, or in raising the
road-bed across low and wet places to provide for
68 A TEXT-BOOK ON ROADS AND PAVEMENTS.
(3) The construction of artificial drainage where a
road is inclined to be wet, as already discussed in
(4) Improvement of the surface, which may consist
in reforming the surface of natural earth, or in the
construction of an artificial surface or pavement, the
latter of which will be discussed in separate chapters.
The problem in common-road improvement is for
the most part that of making the most of the roads
that exist, rather than reconstructing them with new
material. The materials and funds immediately avail-
able must be used to secure as much improvement as
Earth roads, under the most favorable circumstances,
do not usually attain any high degree of efficiency,
and are not economical under any considerable traffic.
They are, however, capable of much improvement, and
if properly managed need not become, as they fre-
quently do, practically useless during a large portion
of the year, although they are always more difficult
and expensive to maintain in a good condition than
roads of a better and more permanent construction.
ART. 25. EARTHWORK.
Improvements to the road-bed of an existing coun-
try road may have for their object the reduction of
gradient upon steep inclinations, by cutting the ma-
terial from the road-bed and lowering the surface of
the road on the upper part of the grade, and filling in
correspondingly on the lower part, or they may be
intended to provide better drainage by raising the
road across low ground.
IMPROVEMENT OF COUNTRY ROADS. 69
In the construction of new roads, the formation of
the road-bed consists in bringing the surface of the
ground to the grade adopted for the road. This grade
should be carefully established upon an accurate pro-
file of the line, in such manner as to give as little
earthwork as possible, both to render the cost of con-
struction low, and to avoid unnecessarily marring the
appearance of the country in vicinity of the road.
The most desirable position of the grade line is
usually that which make the amounts of cut and fill
about equal to each other, especially where room for
borrow-pits, or spoil-banks, would be expensive, anc
it is desirable to make the embankment for the most
part of the material taken from the road excavations.
On side-hill work, one side of the road is commonly
in cut and the other in fill, and where the side slopes
are steep, it is usually better to make the road mostly
in cut on account of the difficulty of forming stable
embankments on steep ground.
Where embankments are to be constructed, the sur-
face of the ground should be cleared of all vegetable
matter and soft material before beginning the placing
of the earth-filling, in order to give a firm base to the
bank and permit it to bond with the earth below.
The material of an embankment should be as homo-
geneous as possible, and all perishable matter should
be carefully excluded from it. It should be deposited
by beginning at the outside and working toward the
middle in such a way as to give a concave section to
the top of the bank during construction, which tends
to prevent sloughing off along the lines of the joints
between the various layers. It is also best to build an
embankment a little narrower at bottom
70 A TEXT-BOOK ON ROADS AND PAVEMENTS-
at top than it is intended to remain, and afterward
trim down the edges to the proper slope.
Earth in an embankment will compact closer than
it is found in the natural state. On an average it will
shrink about one tenth of its bulk. The allowance to
be made for settling in forming an embankment de-
pends upon the method of construction. Where scrap-
ers are used, the earth will usually be well compacted
in placing, and no allowance is necessary ; with dump
carts or wagons the compacting is not so thorough,
and a small allowance should be made ; while when
wheelbarrows are used or the earth is thrown into
place with shovels, an allowance of 10 or 12 per cent
must be added to the height of the embankment, in
order to allow for the final shrinkage. Rock occupies
more space in embankment than in excavation, and
does not need allowance for shrinkage.
In constructing embankments across wet and un-
stable ground, it is frequently necessary to form an arti-
ficial foundation upon which to place the earth em-
bankment. This may be accomplished in some cases
by excavating a little of the soft material and substi-
tuting sand or gravel, or in other cases it may be
advisable to employ layers of brushwood or fascines
as a support for the embankment. Sometimes it may
be possible to drain the soft material by deep ditches,
so as to render it capable of sustaining the road, and
in all cases drainage should be provided in so far as
possible to make the embankment more secure.
When embankments are to be formed on sloping
ground, the surface of the ground should be stepped
off, in order to hold the earth-filling from sliding upon
the natural surface at the line of contact between the
IMPROVEMENT OF COUNTRY ROADS. Jl
two, until it becomes sufficiently settled for the de-
velopment of cohesion to cause it to become one solid
In many cases where roads are to be constructed
along steep slopes, it is found cheaper to use retaining
walls to sustain the road upon the lower side and the
earth-cutting on the upper side than to cut long slopes
or form high embankments.
Catch-water drains are necessary on the natural sur-
face above the top of all high slopes in cuttings to
prevent the surface-water from washing down and
destroying the face of the slope.
Where springs are tapped by a cutting, drains must
be provided to remove the water without injury to
the slope ; and where the subsoil may become wet in
rainy weather, it may be necessary to provide sub-
surface drains along the slope to prevent the earth
becoming saturated and sliding down into the road-
Slopes, both of excavation and embankment, are
greatly improved by being sodded or sown with grass.
This aids in the maintenance of the slopes, by render-
ing them more capable of resisting the abrading action
of such water as falls upon them. It also greatly im-
prove^ their appearance.
The most important principle involved in the forma-
tion of a road-bed, which should be always in mind, is
that earth in order either to sustain a load or to main-
tain a slope, must be kept dry, or at least prevented
from becoming saturated with water, as both the
cohesive and frictional resistances of earth are dimin-
ished or destroyed when it becomes wet, and it is
also then liable to the disturbing action of frost.
72 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 26. DRAINAGE.
Drainage is especially important upon earth roads,
because the material of the road surface is more sus-
ceptible to the action of water, and more easily
destroyed by it than are the materials used in the
construction of the better class of roads. When
water is allowed to stand upon the road, the earth is
softened and readily penetrated by the wheels. The
action of frost is also apt to be more disastrous upon
the more permeable surface of the earth road, having
an effect to swell and heave the roadway and throw
its surface out of shape. It may in fact be said that
the whole problem of the improvement and mainte-
nance of ordinary country roads is one of drainage.
In underdraining an earth road on account of the
permeability of the surface, provision must be made
for carrying off the water which penetrates through
the surface, as well as that due to natural wetness of
the subsoil. The surface should of course be made of
such form and material as to cause the water to flow
off without penetrating as far as possible ; but in
damp weather wheels will mark the surface somewhat,
and water held in the ruts so formed will soak into the
earth, and unless at once removed below soften it so
that the next wheel makes a deeper rut, with the
final result of destroying the form of the road as well
as its power to sustain the loads that come upon it.
The necessity for the application of artificial sub-
drainage in any case is determined by local conditions,
the character of the soil, and natural drainage. An
examination of the line of the road in wet weather,
observing whether water stands upon the ground, the
IMPROVEMENT OF COUNTRY ROADS. 73
direction of flow of surface-water, and whether that
which penetrates the ground drains away quickly is
usually an efficient aid in forming an opinion as to the
necessity for drainage.
The methods employed in draining are considered
in Chapter II. Dependence is most commonly placed
upon shallow side-ditches, which are seldom of much
value except to carry off surface-water ; and even when
the side-ditches are deep, they can only be efficient
for subdrainage when the soil is of a very open,
porous nature. In other cases they will not draw the
water from the subsoil under the middle of the road,
and cross-drains or a centre drain should be provided.
The common neglect of proper drainage is undoubt-
edly very largely responsible for the general bad con-
dition of country roads.
ART. 27. EARTH-ROAD SURFACE.
The method which should be adopted for the im-
provement of the surface of an earth road depends
upon the nature of the material of which it may be com-
posed. When the material is loose sand, the surface
will be more firm if the sand be damp and more
unstable in dry weather. In such case a small admixt-
ure of clay in the surface layer may give cohesion to
the surface when dry, or a layer of clay six or eight
inches deep may form a hard and comparatively
durable surface, as it is easily drained when upon the
Clay soils as a rule absorb quite freely the water with
which they may be held in contact, and soften when
saturated, but are not readily permeable, and hence are
74 A TEXT-BOOK ON ROADS AND PAVEMENTS.
not easily drained from below. Used alone they are
consequently the least desirable of road materials.
When dry, clay may make a very hard and durable sur-
face, and it may give good results as a covering for a road-
bed of more pervious material, or it may form a stable
road-bed when protected by a surface which does not
soften so readily and prevents the surface-water from
reaching the clay beneath. In building over clay, sand
or gravel may frequently be mixed with the clay to
form a surface layer which will be less acted upon
by water. When rather coarse sand or small gravel is
used for this purpose and a small proportion of clay
just sufficient to bind the particles of sand together,
a very hard and compact mass is formed, nearly im-
pervious to water and but little acted upon by it. Ma-
terial of this nature found in a natural state is known
as hard-pan, and is very stable and durable. A layer
of sand a few inches deep may also sometimes be
employed to form a surface over a clay road-bed which
will not soften in wet weather, and will afford protec-
tion to the clay beneath.
When other material cannot be obtained, clay roads
are sometimes improved by burning the clay so as to
form a more porous material for use as a surface layer.
This method, however, is somewhat expensive, and
other materials may usually be employed at less cost.
Soils composed of mixtures of sand and clay or of
gravel and clay are usually easier to deal with than clay
itself, and commonly form the best natural roads. They
vary in character from the light sandy loams to heavy
soils partaking very much of the nature of clay. The
sandy soils take up water readily and become soft when
wet ; but they are pervious and easily drained, and they
IMPROVEMENT OF COUNTRY ROADS. 75
may be compacted into a firm surface in dry weather.
The heavier soils take up water readily and become soft
when wet, but are less pervious and drained with more
difficulty, though much more easily than a clay.
The material of a road surface should always be such
as may be compacted to a firm and hard surface.
It should not, therefore, be formed of the soft ma-
terial which may be washed into the gutters. The sur-
face must be formed with a crown at the middle suffi-
cient to shed the water which may fall upon it into the
gutters, and prevent water from standing upon the
road. The slope necessary to shed the water readily
is about i in 20, and the most desirable section is usu-
ally that composed of two planes of equal inclination,
rounded off in the middle and sloping uniformly to the
sides, as shown in Fig. 16.
In the construction of an earth-road surface, road-
machines or road-scrapers may often be employed to
advantage, especially when no grading is to be done
other than giving the road the proper crown. The gut-
ters may thus be formed, and the surface shaped up
with comparatively little labor.
After the material is in position, the surface should be
compacted to the required form by rolling with as
heavy a roller as may be available. This is a very im-
portant matter in attempting to form a satisfactory
earth road, and is almost indispensable to success. If
the loose earth be thrown into the middle of the road
to be compacted by the wheels of traffic, the action of
76 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the wheels will be to cut it, or at least to pack it in a
very uneven manner, producing a surface uneven and
full of ruts, which will hold water and ultimately cause
the destruction of the road. In case, however, the sur-
face be properly rolled, it may usually be made suffi-
ciently firm to hold up the wheels and retain its form
under the traffic, and if kept free from ruts until thor-
oughly compacted will thus be rendered much more
capable of resisting the penetration of water and shed-
ding it into the side gutters.
ART. 28. GRAVEL ROADS.
Gravel roads may vary from that in which a thin
coating of gravel is used as a wearing-surface upon
an earth road to that in which gravel is used as a sur-
face for the heavy Telford construction of a road of the
first class. These latter constructions will be treated in
Chapter V, under the head of " Broken-stone Roads."
In the improvement of a country road, where the
construction of a good Telford or Macadam road can-
not be undertaken, a surface of gravel may frequently
be used to advantage, giving much better results than
could be obtained with the surface of earth. Even
a light layer of gravel may frequently prove of very
Where the subsoil is of a porous nature and well
drained, a layer of three or four inches of gravel, or some-
times even less, well compacted, will constitute a very
considerable improvement ; especially if, as is usual with
these light soils, the nature of the material of the road-
bed is particularly unsuitable for the wearing-surface,
difficult to compact sufficiently to shed water, and likely
to become soft when wet.
IMPROVEMENT OF COUNTRY ROADS. 77
Where the road-bed is of clay a deeper layer of
gravel, at least 6 inches, is usually required for effective
work, as the gravel must be deep enough to prevent
the weight of the traffic forcing the surface layer into
weak places in the clay beneath, and also to effectually
prevent the surface-water from reaching the clay.
Gravel to be used on roads should be sharp and
comparatively clean. In order to bind well in the road
it should usually have a small admixture of clay.
Gravel in which the stones are round or oval, such
as is commonly found in the beds of streams, is unfit
for the construction of roads; the small stones of which
it is composed, having no angular projections, will not
bind together, and even when mixed with clay may turn
freely, and will be difficult to firmly bed in position.
Pit gravel is usually more sharp, but is frequently found
mixed with considerable earth, which, as well as the
larger stones should be removed by screening before
using the gravel. Screens of ij inch and J inch open-
ings may be employed for this purpose that material
only which passes the larger and is rejected by the
smaller being used in the work.
In the construction of a road with gravel surface the
road-bed should first be brought to the proper grade,
with a form of cross-section the same as that to be
given the finished road. The gravel is then placed upon
it and rolled to a surface, or left to be compacted by
the traffic. It is always advantageous when possible to
compact the road by rolling. The road-bed should be
well rolled before placing the gravel, and the gravel
surface afterward. A smooth hard surface may thus be
produced, upon which the wheels of loaded vehicles
may roll without producing any visible impression.
78 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Where the compacting of the road is left to the traffic
constant watchfulness is necessary to prevent unequal
wear and the formation of ruts.
ART. 29. MAINTENANCE OF COUNTRY ROADS.
The maintenance of a country road in good con-
dition is a matter requiring constant care and watch-
fulness. Any small breaks in the surface must be
immediately repaired, and ruts filled and smoothed
before they become serious.
The work required to keep a road in repair depends
upon the nature of the surface and the efficiency of
the drainage. A well-constructed road of good ma-
terial will be much easier and less expensive to keep in
repair than one in which the surface is not firm enough
to resist the cutting action of the traffic, or which has
a surface compound of material readily softened by the
action of water which may fall upon it.
Earth roads under the most favorable conditions are
expensive to maintain, and especially so under the
common system of repairing once or twice a year, or
at long intervals. This system is not only costly in the
work required, which usually amounts to a practical re-
construction of the road each time repairs are under-
taken ; but it is ineffectual in that the road for the larger
portion of the time is out of repair and in bad condi-
tion, even if the work of construction has been well
done, which is not usually the case where this method
The only way to keep an earth road in good con-
dition is by the employment of men whose business it
shall be to continually watch the road, and make such
IMPROVEMENT OF COUNTRY ROADS. 79
small repairs as may be necessary from time to time.
The small washes that may occur during heavy storms,
ruts formed by wagons travelling in the same track, or
in passing over soft spots when the road is wet, or
any small breaks in the surface of the road, should be
at once attended to and carefully filled with new
Where small repairs are needed over a considerable
area of the road the use of the road-machine is usually
advantageous, as giving an easy method of smoothing
up the surface. The use of a roller is also nearly
always of value, both to assist in smoothing the surface
to the proper form, and to give compactness to it. By
the occasional use of these machines through the dry
seasons a road may be kept crowning and hard, so that
most of the rainfall will be quickly shed off into the
side gutters without injury to the road.
When there are long-continued rains, or when the ice
and snow of winter are melting in the spring, an earth-
road surface will necessarily be more or less softened
and cut by passing vehicles ; and at such times a road
of this character cannot be maintained in the same
condition as in dry weather, or in the condition which
would be possible with a less permeable surface, but if
at the beginning of the wet period it be in proper form
and if the subdrainage be efficient, the injury to the
road as well as the duration of the bad condition, will
be reduced to a minimum. As soon as possible after
such a wet time, the roads should be gone over with
the scraper and put into proper form, and then rolled
down hard. It is advantageous to have this done be-
fore the ground becomes thoroughly dry and hard, as
80 A TEXT-BOOK ON ROADS AND PAVEMENTS.
it will work more freely, and may be compacted much
closer by the roller than afterward.
In repairing a road where the gutter is filled with
soft material which must be removed to afford a free
channel for the surface-water, this soft material should
not be scraped upon the middle of the road, as it will
not form a good wearing-surface. Where, however,
a road is in fairly good condition, and merely needs
a little smoothing up, it is desirable to work from the
gutter, scraping the material lightly toward the middle
until the proper crown is obtained.
The difficulty and cost of maintaining a road will of
course vary with the nature of the traffic that passes
over it. A road for light driving will be much easier
to keep in repair than one used by heavy loads, and
as the amount of heavy traffic becomes greater the
economy of the earth-road surface is lessened, and the
desirability of the substitution of a more durable wear-
The width of the wheel-tires upon which the loads
are carried is also important in its effect upon the cost
of keeping a road in repair. Narrow tires cut and rut
the surface of a road, while those of sufficient width act
as rollers to compact the material. For the best results
the tires should be as wide as possible, and the front
and rear wheels of a wagon should not run in the same
track. The lighter tractive effort required for wide
tires on compressible road surfaces has been referred
to in Art. 2.
ART. 30. WIDTH OF COUNTRY ROADS.
The width of the roadway upon country roads
should be only sufficient to provide space for the easy
IMPROVEMENT OF COUNTRY ROADS. 8 1
conduct of the traffic. For roads of ordinary traffic
this requires only that there shall be room for teams
moving in opposite directions to freely pass each
other. An available width of 16 feet is ample for this
purpose, and 14 feet is often sufficient. Too great
width in the roadway causes an unnecessary increase
in the cost of constructing and maintaining the road.
Where the road-surface is of earth it will be much
easier to drain it if it be narrow than if it be wide. If
deep side-ditches be depended on for subdrainage, the
nearer they are together the more effectively will they
drain the subsoil under the middle of the road. Side
ditches must, however, be far enough apart so that a
berm may be left on each side between the travelled
part of the road and the ditches. Thus in Fig. 7, p. 31, if
the macadamized portion represents the travelled part
of the road, the berm between that portion and the
ditches could be sown with grass and show the line of
the road as a guide to travel.
When covered drains are used for subdrainage the
gutters at the side may be made shallow and placed
next the travelled part of the road, giving much less
surface to maintain and greater efficiency to the drain-
age than in a wider road. Such sections are shown
for earth roads in Figs. 6 and 16. Fig. 4 shows a
similar construction with side-drains under the gutter
and a broken-stone surface. Fig. 17 shows the
ordinary form of a country road with broken-stone
surface. On important roads the paved portion is
commonly 16 or 18 feet in width, but on roads of
lesser importance it may be less, and under light traffic
a width of 10 feet maybe sufficient teams, when i
sary, turning out upon the sod to pass. Where^less
82 A TEXT-BOOK ON ROADS AND PAVEMENTS.
pervious covering is employed, as with gravel or broken
stone, width will not have the same tendency to render
drainage ineffectual as in the case of an earth road,
because comparatively little water will pass through
the road-surface to the subsoil. The cost of main-
tenance may not, therefore, be so materially affected
by the width, although the cost of construction, and
hence the length of road, that may be built for a give
sum will be directly dependent upon it.
While the improved portion of the road should be
as small as is consistent with the proper discharge of
the duty required of it, the available right of way need
not be so restricted, but should be laid out wide
enough to permit of the widening of the used portion
when necessary, and allow room at the sides for pedes-
trians, with a grass border and line of trees. When
trees are planted along the roadway they should not
be placed so as to form a dense shade over any portion
of the travelled road, although a moderate shade is
not a disadvantage, and care should be used that they
are not near enough to a covered drain to permit the
roots to grow into the drain and choke it.
ART. 31. ECONOMIC VALUE OF ROAD IMPROVEMENT.
The value of a road improvement to a community
and the amount of money that may reasonably and
profitably be expended in the construction and main-
tenance of common roads is a subject the discussion of
IMPROVEMENT OF COUNTRY ROADS. 83
which leads different persons to widely different con-
clusions, depending upon the point of view and the
The economic principles involved in a choice of
location have already been discussed in Chapter III ;
and the general value of any other improvement, in so
far as it relates to the economic conduct of the traffic,
may be considered in the same manner. Any improve-
ment, either in position or surface, that has the effect
of increasing the loads that may be taken over a road
by a given power lessens the number of loads neces-
sary to carry the traffic, and effects a saving in time
and labor of men and teams, which may reasonably be
considered to have the same money value as the time
used in the work.
On ordinary country roads in dry weather, the amount
of load that can be hauled is usually determined rather
by the grades than by the nature of the surface. Un-
less the gradients are very light the amount of load
that can be carried on a broken-stone surface does not
differ greatly from what may be taken on a dry and
hard earth road. In improving a road by substituting
a hard surface for a surface of earth the gradients and
location should therefore always be carefully studied,
with a view to deriving the full practical benefit from
the hard surface in the light traction that it may re-
quire with easy ruling gradients.
It is in wet and muddy weather that improved sur-
faces have their chief advantage over earth roads, and
the main object of introducing hard and impermeable
surfaces is to eliminate the period when ordinary earth
roads are apt to be muddy and practically useless for
the purposes of transportation, and to substitute a
84 A TEXT-BOOK ON ROADS AND PAVEMENTS.
road that may be used at any season. Systematic
drainage has a similar object. To a farming com-
munity the economic advantage of a road uniformly
good at all seasons is greater than might appear at
first glance. It may in many instances amount prac-
tically to a saving equal to nearly the entire cost of
hauling, by permitting the work to be done at times
when other work is impossible, thus making men and
teams available for other duty in good weather. The
ability to use a road at any season is also of advantage
in the independence of weather that will make it pos-
sible to take advantage of the condition of the markets
in the disposal of produce or purchase of supplies.
The nature of the roads has likewise an important
effect upon the social life of the people in a rural dis-
trict, and has much to do with the desirability of a
locality as a place of residence. These items all have
a real importance, which, while difficult to estimate in
money values, show at once in the fact that prices of
country property are largely affected by them.
The nature of the country roads affect the towns to
which the country is tributary as well as the country
itself. They directly affect trade in seasons of bad
weather, both in regulating the demand for supplies
for country consumption and in controlling the supply
of produce which is available for market ; indirectly
also the prosperity of a rural district means that of its
All of these points must be considered in any at-
tempt to arrive at any proper conception of the advan-
tages of a proposed improvement. In any particular
case the local interests will determine the relative im-
portance of the various elements, and a careful analysis
IMPROVEMENT OF COUNTRY ROADS. 85
of the trade that does pass over the road and that
would pass over it under different conditions will
enable a judgment to be formed as to the value of
The money spent in road improvement is to be con-
sidered as an investment, which will return annual
interest to the community in reduced costs of trans-
portation and greater freedom of traffic and travel.
ART. 32. SYSTEMS OF ROAD MANAGEMENT.
Several different systems for managing the work of
constructing and repairing country roads have been
proposed or are in use in various places. These sys-
tems differ in the placing of the control of the roads
and in the methods adopted for providing funds.
The control of the roads under the various systems
may be vested in the national government, in the vari-
ous State governments, in county or parish organiza-
tions or in townships or districts. In regard to the
location of control and responsibility, it may be re-
marked that there are two points to be kept in view.
ist. In order that the work may be economically
conducted, the section of country included under one
control should be sufficient to warrant the permanent
employment of a man, or corps of men, whose business
it shall be to continually look after the roads, study
their needs, and systematically conduct their improve-
ment. It should admit of the ownership and use of
labor-saving machinery for the economical execution
of the work, but should not be large enough to require
an elaborate and complicated organization.
2d. The control of road work should be so arranged
that, as nearly as possible, all of the interests directly
86 A TEXT-BOOK ON ROADS AND PAVEMENTS.
affected by the condition of any road shall have a
voice in its management and contribute to its support.
Common roads are essentially local in their character
and are not usually employed as lines of continuous
transportation over any considerable distance. They
are not, therefore, of State or national importance as
lines of communication, although as factors in the
general welfare of the people they must, of course, like
all other such factors, be of general interest and con-
cern to both State and nation.
The nation, and in most cases in this country the
State, is too large an unit to assume direct control of
road work. In general, the interests over so large an
area are so varied, and the requirements so different,
as to prevent a harmonious and successful organization
of such work with a probability of economical adminis-
tration. In some cases, however, such control might
be wise and proper, and the recognition of the impor-
tance of road improvement to the general welfare of
the State, through the payment by the State of a
portion of the cost of permanent improvements, has in
some instances proved a powerful stimulus to local
The control of road management by towns and small
districts is nearly always inefficient because the organ-
ization is too small to support a proper management
or provide the necessary appliances for economic work.
Under this system the man in charge of the roads is
usually engaged in other work, he is not a road engineer,
and can, and is expected to, give but little attention to
the road work. This system of control is also usually
unfair, except in case of roads intended for the accom-
modation of the local district only. For instance, a
IMPROVEMENT OF COUNTRY ROADS. 8?
road passing through a town may be a thoroughfare
for the towns upon each side. The principal traffic
may be this through-trade to points beyond the limits
of the town in which the road is situated. The cost of
keeping up this road is largely due to outside traffic,
and the intermediate town should not be required to
bear all the expense of maintenance. On the other
hand, the interests of the towns whose trade passes
over the road are largely affected by its nature, and
the people of these towns should be permitted a voice
in determining the character of the road. Most of the
more important roads of every vicinity pass thus
through several towns, and the system of improvement
by small districts works injustice both ways upon
those who are obliged to keep a road for the use of
others and upon those who are obliged to use a road
they cannot cause to be kept in proper condition.
County management seems more successful in this
country than any other, as a county, or two counties
combined if necessary, is usually strong enough to
secure intelligent management and homogeneous
enough to have common interests.
The proper management of the common roads in
any community requires both experience and intelli-
gence. A man to be efficient in such work must be
able to make or modify location where necessary,
judge of the value of various materials for purposes of
construction, determine the necessity for and means to
be adopted for drainage, and possess the executive
ability to manage men and control scattered work.
The work In each locality is a problem by itself, to be
solved by careful study of the requirements of the
community, taking into account the local natural con-
ditions and available materials and means.
ART. 33. DEFINITION.
BROKEN-STONE roads consist essentially of a mass of
angular fragments of rock deposited, usually in layers,
upon the road-bed or a foundation prepared for it, and
then consolidated to a smooth and uniform surface by
means of a roller or by the action of the traffic which
passes over it.
There are two commonly recognized systems of con-
structing broken-stone roads, differing in the nature of
the foundation employed, and known respectively by
the names of the men who first introduced them into
English practice as Telford roads and Macadam roads.
Each of these systems has been greatly modified in
use since the time of its founder, and each name is now
used to cover a general class of constructions differing
very materially within itself as applied in the practice
of different engineers. Each of the systems also has
its earnest advocates, who contend for its exclusive use,
and numerous controversies have been the result, at
the conclusion of which each party is " of the same
opinion still." The view taken by different road-
builders in this matter, it may be remarked, appears to
be the result usually of the local necessities of the
vicinities in which they work, and of the skill with which
BROKEN-STONE ROADS. 89
the different systems have been applied in work which
has come under their observations. In road-building,
as in any other class of engineering works, no rigid
rules can be laid down for universal application ; each
road must be designed for the place it is to occupy and
the work it is to do.
In some parts of this country natural gravel is sub-
stituted for broken stone in the construction of these
roads, the methods of construction being the same as
in using broken stone.
ART. 34. MACADAM ROADS.
Macadam roads as commonly constructed consist of
two or more layers of broken stone, each layer being
rolled to a firm bearing before placing the next. The
broken stone is usually placed directly upon the earth
In constructing a macadamized roadway, the road-
bed is first brought to the proper grade in the usual
manner, and rolled to a uniform surface. The surface
of the road-bed is either flat or raised at the middle to
the same section as is to be given the finished road-
surface. The inclined form is usually employed, and
seems preferable on account of affording better drain-
age in case any water finds its way through the surface
On village streets where curb and sidewalks are em-
ployed, this section of the road-bed may extend to the
curbing (as shown in Fig. 5^, but on country roads a
bench of earth should be left at the side between the
broken stone and the gutter in order to confine the
broken stone while it is being compacted, and of-event ^
90 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the spread of the surface materials. The form of the
road-bed before placing the stone would then be as
shown in Fig. 18, where the completed road is to be of
the form given in Figs. 4 and 7. Where the road-bed
is in embankment, it is common to construct the earth
embankment to the height of the finished surface, and
afterwards excavate the material necessary to admit of
placing the surface layers. The embankment should
be allowed to settle and become thoroughly compacted
before the broken stone is placed upon it, and it is
desirable with new embankments that they be used for
a short time by the traffic upon the earth surface be-
fore finishing the road ; where, however, the material
is well compacted in construction and can be thor-
oughly rolled this is not necessary.
In constructing the road-bed its proper drainage
must be considered, and where necessary to prevent its
becoming wet under the broken stone some means
should be adopted to artificially drain it.
Upon the completion of the road-bed, a layer of
broken stone, usually from 3 to 5 inches in thickness, is
placed upon it and thoroughly rolled. Upon this a
second layer is placed and likewise rolled to an uniform
surface. Sometimes a third layer is added, or in case
of a very thin road it may consist of a single layer, the
number of layers depending upon the thickness of the
road. When no roller is used, the stone is usually
spread on the surface of the road-bed to the full thick-
BROKEN-STONE ROADS. 9!
ness desired for the road, and left to the action of the
The upper layer constitutes the wearing surface of
the road, and upon this it is usually necessary to place
a thin layer of finer material called binding material,
which may consist of rock chips, sand, small gravel, or
sometimes loam, and is washed and rolled into the inter-
stices of the rock, with the object of forming a com-
pact and impervious surface. Binding material is in
like manner often added to the lower layers of the
road, although this has not been common practice.
The object should be to fill the voids in the rock as
completely as possible, serving to make the road one
solid mass, to bind the rock more firmly together, and
to prevent the percolation of water through the surface.
When a road is to be constructed over a heavy soil
not easily drained and apt to be wet and soft, a foun-
dation consisting of a thin layer of sand or gravel may
frequently be employed to advantage. This founda-
tion layer will serve to prevent the stones of the lower
stratum of macadam from being forced downward into
the soft material of the road-bed, or the material of the
road-bed from forcing upward into the interstices of
the broken stone. This foundation may consist of a
layer of sand or gravel from 2 to 5 inches thick, and
should be well compacted by rolling before the placing
of the broken stone.
ART. 35. TELFORD FOUNDATIONS.
The distinguishing feature of a Telford road is its
paved foundation. It consists essentially of a pave-
ment of stone blocks set upon the road-bed and cov-
ered with one or more layers of broken stone.
92 A TEXT-BOOK ON ROADS AND PAVEMENTS.
In forming a Telford road the road-bed is con-
structed in the same manner as for macadam, being
made either level or crowned. A pavement is then
placed upon the road-bed from 5 to 8 inches thick, de-
pending upon the thickness to be given the road
material, the general practice being to make the pave-
ment about two thirds of the total thickness of the
road. The stones used for the pavement may vary
from 2 to 4 inches in thickness and 8 to 12 inches in
length ; they are set upon their widest edges and with
their greatest lengths across the road. The irregulari-
ties of the upper part of the pavement are then broken
off with a hammer, and all the interstices filled with
stone chips and wedged with a light hammer so as to
form a completed pavement of about the thickness re-
Upon this pavement the layers of broken stone are
placed, and the road-surface completed in the same
manner as for a Macadam road.
The practice of Telford was to grade the road-bed
flat, and then construct his pavement deeper in the
middle than at the sides, using for a roadway 16 feet
wide stones about 8 inches deep at the middle and 5
inches at the sides. This practice is still followed by
some engineers, but it is now more common and usually
considered preferable to make the surface, of the road-
bed parallel to the finished surface and the pavement
of uniform thickness. Fig. 19 shows a section of Tel-
ford road as now commonly constructed.
Some engineers in constructing Telford foundations
'do not roll the road-bed, but simply bring it to grade,
and then lay the pavement by bedding the stones in
the surface of the road-bed sufficiently to bring their
BROKEN-STONE ROADS. 93
tops to the proper height, in which case it is unneces-
sary to trim off the tops with the hammer as in the
An objection sometimes urged against the Telford
foundation is that if the foundation be of hard stone it
will cause the material above to be crushed by the loads
which come upon it, and that greater durability in the
wear of the road metal will be obtained by having a
more yielding foundation. The durability of the Tel-
ford road has, however, been established by long-con-
tinued usage. There is no apparent reason why a firm
foundation should cause greater wear at the surface,
and the materials below the surface are never crushed
in the destruction of any broken-stone road.
The relative value of the two systems must always
be determined by the local conditions under which a
road is to be constructed and the necessity for such a
foundation in the particular case.
ART. 36. CHOICE OF FOUNDATION.
The proper foundation to be used for a broken-stone
road depends upon the nature and condition of the
road-bed upon which it is to be constructed and the
nature of the traffic to pass over it. If a firm, well-
compacted, and thoroughly drained road-bed may be
obtained, of material which will not readily soften
94 A TEXT-BOOK ON ROADS AND PAVEMENTS.
under the action of moisture, there will usually be no
need for a special foundation, but the first layer of the
macadam may be placed directly upon the surface of
the road-bed. If, however, the road-bed is of a ma-
terial retentive of moisture, not thoroughly drained,
and likely to become soft in wet weather, and the
broken stone be laid immediately in contact with it,
the stones of the lower layer of macadam may be grad-
ually worked down by the weight of the traffic into the
soft earth, and the soil at the same time work up into
the voids in the stone, causing a gradual disintegration
of the road. It may thus also become retentive of
moisture and subject to the disrupting action of frost.
In this case some foundation must be provided which
is capable of resisting the penetrating action of the soft
material of the road-bed, and of distributing the load
over it. This may be the Telford foundation as de-
scribed in Art. 35, the sand or gravel foundation men-
tioned in Art. 34, or the Telford foundation upon a
layer of sand or gravel, depending upon the extent of
the difficulty to be met.
It is not intended in the above to imply that the use
of a foundation of this character should take the place
of proper drainage. The advisability of artificial
drainage should always be carefully considered, and
where the road is threatened by water which may be
removed by the construction of drains they should be
used, but frequently thorough drainage is difficult or
doubtful, and it is desirable to adopt heavy construc-
tion such as the Telford foundation gives.
iTi some instances it may be possible, by drains under
the road and substituting porous material immediately
under the broken stone, to use light macadam super-
BROKE X-STONE ROADS.
structure and do away with the necessity for the Tel-
ford pavement in difficult soils. Thus in Fig. 20 a con-
struction is shown applicable to wet and unstable soils,
the space over the centre-drain and under the middle
of the macadam being filled with large rounded stones,
which secure drainage and form a stable bed for the
It is commonly claimed by the advocates of the
Macadam system of construction that on any well-
drained and well-compacted road-bed there will be no
tendency on the part of the stone to work down or of
the soil to work up, and hence that the Telford foun-
dation is an unnecessary expense. The difficulty of
procuring a perfectly stable and reliable road-bed in
many localities is, however, very generally recognized,
and Telford pavements are largely used.
It would undoubtedly be an advantage in the con-
struction of any broken-stone road, either Macadam or
Telford, to have a layer of sand or gravel between the
road-bed and the pavement, both as assisting drainage
and as providing against unequal settling of the foun-
dation of the road. The application of it, however,
must in any case depend upon its cost and its apparent
necessity. In cases where drainage is difficult and the
96 A TEXT-BOOK ON ROADS AND PAVEMENTS.
soils inclined to be damp and soft, it may frequently
prove the simplest solution of the problem.
Concrete foundations are often recommended for
broken-stone roads, and would undoubtedly be very
beneficial in most cases, but usually where so expensive
a foundation may be employed a better surface might
advantageously be used than broken stone. It may
sometimes occur, however, that, in places where the
foundation is difficult to maintain, a light bed of con-
crete may prove of great benefit, as forming a firm and
impervious base to rest upon damp and unstable soils.
ART. 37. MATERIALS EMPLOYED.
A stone to be durable in the surface of a road should
be as hard and tough as possible. The qualities of
toughness and resistance to abrasion are of more im-
portance than hardness and resistance to crushing. A
stone may be hard and brittle, and quickly pound to
pieces in a road surface, or it-may have a high crushing
strength and grind away quickly under abrasion, as is
the case with some varieties of sandstone. If, how-
ever, it be too soft, it may crush under the loads com-
ing upon it, and thus lack in durability.
A stone for a road-surface must also resist well the
disintegrating influences of the atmosphere. It should
be as little absorptive of moisture as possible in order
that it may not be liable to injury from the action of
frost. Many limestones are objectionable on this ac-
Basalt and syenite are, in general, the best materials
for this purpose. The harder limestones in some lo-
calities make a good and durable surface. Soft lime-
BROKEN-STONE ROADS. 97
stones crush under the action of the wheels, and soon
become dust and mud. Sandstones as a general thing
are not fit for this use.
The material of a road surface should also be uni-
form in quality; otherwise the wear of the surface will
not be even, and depressions will appear where the
softer material has been placed.
As the under parts of the road are not subject to the
wear of the traffic, and have only the weight of the
loads to sustain, it is evidently not important that the
foundation or lower layers be of so hard or tough a
material as the surface ; and hence it is frequently pos-
sible, by using an inferior stone for that portion of the
work, to greatly reduce the cost of construction.
A judgment as to the value of any given stone for
road use can ordinarily be formed from what may be
known of its behavior under other uses to which it may
have been subjected, or from its appearance where it
has long been exposed to the weather, together with
such physical tests as may be necessary to determine if
it possesses the special properties desired.
Tests may be made of the power of absorption by
drying a sample, weighing it, then placing it in water
and reweighing occasionally until it ceases to gain in
weight. The absorption may then be expressed as a
percentage of the dry weight. The resistance to abra-
sion may be found either by grinding a sample upon a
polishing-disk, or by rattling blocks of the material in
an abrasion-cylinder with pieces of iron, and noting
the loss of weight in each case, and comparing such
losses with those of a stone of known value under like
conditions, These examinations with tests of the
crushing strength of the material will enable an ap-
98 A TEXT-BOOK ON ROADS AND PAVEMENTS.
proximate idea to be formed as to the probable wear-
ing qualities of the stone.
The best test of enduring properties will always be a
knowledge of the stone as used for any purpose where
it undergoes exposure.
The selection of a stone for road construction will of
course always depend largely upon what is to be ob-
tained in the locality of the work. The importance of
a thoroughly good material in the road surface is, how-
ever, so great in its effect upon the durability and cost
of repairs of the road that it may frequently be found
economical, on roads subjected to a considerable traffic,
to bring a good material a considerable distance rather
than to use an inferior one from the immediate vicinity.
It may also be suggested in this connection that in
many instances railway transportation over a consider-
able distance may be small compared with wagon
transportation over a ::hort distance, and the impor-
tation of good material may add but slightly to the
aggregate cost of the work.
The size to which stone should be broken for road
material depends to some extent upon the nature of
the material. The harder and tougher it is the smaller
the pieces may be without danger of crushing or shat-
tering under the loads and shocks received in the road
surface, and the smaller also they will need to be in
order to be thoroughly compacted in the road.
It is a general custom to use larger stones in the
bottom courses of a road than at the top. A rule very
commonly given is that the stones for the lower layers
should be at least 2 inches in their greatest diameter,
and not more than 3 inches, and that for the surface
layer the stones shall not be greater
in greatest dimension.
If of very hard rock the surface layer may
inches as an upper limit of size.
The size of the rock in the lower layers does not
seem of so great importance as that for the surface
layers, as it is not directly subject to the weight or the
abrading action of the concentrated wheel-loads, and it
is probable that in some cases unnecessary expense is
incurred in following the refinements of rigid specifica-
tions in this particular.
There is a difference of opinion also among road-
builders as to the advisability of using stone of uniform
size. Some insist quite strenuously upon this point,
and carefully screen their stone with the object of get-
ting it as uniform as possible ; while others declare that
the variation of size is an advantage, and even that the
stone should not be screened after coming from the
crusher, except to remove the stone above the limiting
size and when necessary to get rid of foreign matter
in case it should contain clay or earth.
Uniformity of size probably makes the wear more
even, but the presence of smaller fragments facilitate
the binding together of the material. The best prac-
tice seems to favor the exclusion of the fine material
from the stone, without insisting on too great uniform-
ity in size (stones being allowed probably from J inch
to i or 2 inches in dimension), and then adding small
material after the placing of the stone upon the road
to assist binding.
This eliminates the danger of having portions of the
road composed entirely of fine material.
Road stone may be broken by hand or machine.
IOO A TEXT-BOOK ON ROADS AND PAVEiMENTS.
Hand-broken stone is usually preferred as cleaner and
better in shape for compacting in the road. In England
hand-breaking is largely practised, and it is frequently
asserted that machine-breaking injures the stone by
crushing it in the jaws of the machine with the effect
of decreasing its durability in the road surface.
In American practice machine-breaking is almost
exclusively used. It gives satisfactory results, both as
to binding and durability, and has the advantage of
greatly lessening the cost of construction.
Gravel is frequently used for roads constructed in
the same manner as with broken stone, both with and
without the Telford foundation. The requirements of
a good gravel for this purpose are the same as for a
good stone. The stones of the gravel should be sharp
and angular, and must possess the qualities of hardness
and toughness. Water-worn material is therefore ob-
jectionable, as it will not compact without the use of
large amounts of soft binding material. In many places
a hard flint gravel occurs which is not inferior to the
best broken stone.
Gravel not fit for surface material has sometimes been
used to advantage under a surface layer of hard rock,
and in some cases a surface of flint gravel has been
used upon bottom layers of soft rock.
Gravel should be screened to remove the larger
stones and the fine material, and then treated in the
same manner as broken stone.
Blast-furnace slag may also in some localities be used
as a road material with good results. Slag varies
greatly in its properties, some being porous and brittle,
other hard and tough. The ordinary slag is usually a
good material for foundation and lower layers of tlu:
BROKEN-STONE ROADS. IOI
road ; and where a good, tough slag can be obtained it
may also be used for the surface. In some places slag
is toughened for such use by being cooled slowly under
a cover of ashes or cinders and afterward broken like
Ashes or cinders may also sometimes be employed
as a foundation for a thin-broken stone road. They
serve to secure good drainage and become very com-
pact. In many cases a considerable improvement
might be made in village streets by substituting a layer
of ashes for an earth-road surface, and later a stone
surface could be applied directly to the road so
ART. 38. BINDING MATERIAL.
It was the practice of McAdam to require that all
the stone used upon his roads should be as nearly as
possible of a uniform size, and that no foreign sub-
stance be mixed with it. In more recent practice it
has been found advantageous to use a certain amount
of finer material to fill the interstices between the
stones, and thus aid in the compacting of the road as
well as render it less pervious.
There is considerable difference of opinion upon this
point among road-builders. A few still advocate the
system of McAdam. Others place a thin layer of
binding material upon the surface of the road and
work it into the surface voids, while still others dis-
tribute the binding material through the entire mass of
stone composing the road.
It is agreed that an impervious surface cannot be
formed of blocks of hard broken stone without the ad-
dition of some small material to fill the voids. It has
IO2 A TEXT-BOOK ON ROADS AND PAVEMENTS.
also been found that when the rock is hard, such as is
needed for good wear in a road surface, it will compact
with difficulty, and that a certain amount of binding
material is necessary in order that the road may be
brought to a surface.
The stone forming the body of the road should be
placed and partially compacted before the addition of
the small material, which may then be worked into the
spaces between them.
The office of the binding material is to hold the
stones in place and form a bearing for them, as well as
to prevent the passage of water between them. It has
no duty to perform in sustaining the loads. This is
the objection to having the binding material mixed
with the stones in advance, as would be the case when
unscreened stone is used. A portion of the road stones
would be replaced by small material instead of having
this material only in such voids as necessarily exist be-
tween the stones.
The quantity of binding to be used is that which
will be barely sufficient to fill all the voids in the larger
material. It has been contended that the lower por-
tion of the road should be porous in order to facilitate
the escape of any water that may find its way through
the surface, but the tendency of the best modern prac-
tice is in the direction of filling all the voids as nearly
as possible, thus making the entire road practically one
solid body and it is now commonly agreed that the sur-
face of a properly constructed broken-stone road is very
nearly impervious to water.
The voids in loose broken stone comprise about 40
to 50 per cent of the volume. In the stone when
compacted in the road the voids are somewhat reduced,
BROKEX-STONE ROADS. IO3
probably ranging from 30 to 40 per cent of the volume.
The voids may be approximately determined in any
case by filling a measure with the stone, shaken down
as closely as possible, and then measuring the quantity
of sand that can be added in the same manner.
Binding material may consist of the screenings from
the broken stone used in the road, of sand or small
gravel, or of loam. To produce the best results in
wear the material used for binding should be of equal
hardness with the road stone. Sand, or sand mixed
with screenings, often gives satisfactory results, and
is more easily compacted than screenings alone. With
the use of loam it is much easier to compact the road
to a satisfactory surface than with harder material, es-
pecially if the roller used be a light one. Loam has
been used in some instances with satisfactory results,
but the wisdom of its use is questioned by most
ART. 39. COMPACTING THE ROAD.
The materials may be compected in a road either by
placing them in position and allowing the traffic to
pass over them or by rolling with a steam or horse
The first method by itself is seldom practised when
it is possible to avoid it. It is hard upon the traffic,
takes a long time to reduce the road to a compact con-
dition, and a smooth surface is with difficulty pro-
duced. Where heavy horse-rollers are employed they
are clumsy and inconvenient to handle, and the work
of rolling is slow as compared with the steam-roller.
In many instances, however, good results are obtained
104 A TEXT-BOOK ON ROADS AND PAVEMENTS.
with them. They are not so expensive in first cost as
steam-rollers, and have not the disadvantage of fright-
Some road-builders prefer light-horse rollers of two
or three tons weight, using them either by themselves
or in connection with the travel. In the latter case the
roller simply smooths off the surface as the traffic does
the compacting. (See paper by James Owens, in Trans-
actions American Society of Civil Engineers, Dec.
1892.) These rollers are said to work satisfactorily
where soft binding material is used, although longer
time is necessary than with heavier ones.
Horse-rollers are usually arranged so that the direc-
tion of motion may be reversed without turning the
roller itself around, and also so that the weight may be
changed by placing additional weight inside the roller
or removing it.
Steam rollers weighing from 8 to 15 tons are most
commonly employed for compacting the road materi-
als. They have the advantage of forcing the materials
at once into a firm and compact mass and producing a
smooth surface for the immediate use of travel. They
admit also of the use of hard materials for binding.
In constructing a road with the use of a steam-roller,
the road-stone is first put on to the required thickness
and the roller passed over it to settle the stones into
place and reduce the voids as much as possible. The
binding material, representing a volume about equal to
the voids in the stone, is then added, sprinkled, and
rolled until the small material is washed and forced into
the interstices, giving a smooth, hard surface. This is
repeated for each layer of stone, or in some cases the
small material is only applied to the top layer.
BROKEN-STONE ROADS. 1 05
A thin coating of the binding material is then spread
upon the surface and the road thrown open for travel.
ART. 40. THICKNESS OF ROAD-COVERING.
The thickness nessary for a road-covering depends
upon the amount of the traffic it is to bear and upon the
nature of the foundation afforded by the road-bed.
Under a heavy traffic it is advisable to make the road
covering heavier than might be allowable for lighter
traffic, in order to provide for wear and lessen cost of
When the road-bed is firm, well drained, and not
likely to soften at a wet season, it will always afford a
firm bearing, upon which the covering may rest. The
loads coming upon the road are then simply transmitted
through the covering to the road-bed beneath, and
there is no tendency on the part of the loads to break
through the covering other than by direct crushing of
its material. If, however, the road-bed may become
soft in wet weather, it will then lose its power to firmly
sustain the covering at all points, and ihe covering
must possess sufficient strength to bridge over places
where it is not supported from beneath, or a load com-
ing upon it may break through by bending it down-
ward at such point. The thickness of road covering,
therefore, must be greater where the road-bed is less
The intensity of freezing that may be expected also
has an influence upon the necessary thickness of the
road-covering. The effect of frost upon the road will
depend in large measure upon the condition of the
road-bed, and thus make the thickness depend in still
greater measure upon its nature. Freezing will not
IO6 A TEXT-BOOK ON ROADS AND PAVEMENTS.
injure a dry road-bed, but if it be damp and have but
a thin covering the road is likely to blow or be thrown
up by the action of frost.
For roads on considerable grades the thickness of the
road-covering is often reduced below what is used on
flat ones, because of the better drainage afforded by
the slopes. It is to be remarked, however, that if the
slopes are very steep the wear of the surface becomes
so great, due to the horses' efforts to obtain foothold
and to the washing of surface-waters during rains, that
the thickness of the coating should be increased.
Macadam roads are commonly made from 4 to 12
inches thick, and Telford roads from 8 to 12 inches, of
which 5 to 8 inches may be foundation pavement.
A covering 8 to 10 inches thick is usually considered
ample for nearly any case of a country road, unless
laid upon bad foundation. In case of a slope of more
than 3 or 4 degrees this may perhaps be reduced to 6
inches, or with an especially good road-bed and good
drainage it may even be made 4 inches.
A thin road to be effective must have its interstices
well filled with binding material and be thoroughly
compacted by rolling. It will then present no voids to
be filled by the soil pressing upward from below, and
at the came time it will be practically impervious and
prevent surface-water from reaching the road-bed, thus
keeping the material in good condition to sustain the
loads. The 4-inch roads of Bridgeport, Conn., which
are often cited as examples of successful work, are con-
structed in this manner of exceptionally good mate-
rial. In other cases where thin roads have proved fail-
ures the trouble may often be traced to dampness in
the subsoil or to lack of thorough construction.
BROKEN-STONE ROADS. IO?
In many cases the problem to decide, in determining
the thickness of a covering, is whether to use heavy
construction or thorough drainage. It is easier to get
good results with thick road-coverings, and they are in
general safer to use; but skilful adaptation of less ma-
terial may often save expense in construction with good
results. The peculiar conditions of each case must de-
cide what is best for that case.
ART. 41. CROSS-SECTION.
The side-slopes necessary to enable the water which
falls upon a broken-stone surface to drain off readily to
the gutters is about I in 30 ; hence a crown should be
given the road that will permit that slope. In con-
struction the crown should be made an inch or two
inches, depending upon the width and thickness of the
road, higher than it is intended to remain, in order to
allow for settlement and wear under the traffic.
The arrangement of a cross-section has been referred
to in Art. 30, and sections are shown in Figs. 4, 5, 7, 17,
and 20. In some cases on country roads only the
middle portion of the road is surfaced with broken
stone, and a roadway of earth is left on each side, be-
tween the broken stone and the ditches, upon which
tearris may drive in dry weather. Such a roadway
would commonly be preferred by teams when the earth
is dry and hard, but it renders the road more expensive
ART. 42. MAINTENANCE OF BROKEN-STONE ROADS.
To maintain a brokejvst^iie^road in good condition
it is necessary first^^^taT~ir^p^f^equently cleaned
108 A TEXT-BOOK ON ROADS AND PAVEMENTS.
of mud and dust, and that the gutters and surface
drains be kept open to insure the prompt discharge of
all water that may come upon the surface of the road.
The best method of making repairs that may become
necessary to the road-surface depends upon the char-
acter of the material composing the surface and the
weight of the traffic passing over it.
If the road metal be of soft material which wears
easily, it will require constant supervision and small re-
pairs whenever a rut or depression may appear. Ma-
terial of this kind binds readily with new material that
may be added, and may in this manner frequently be
kept in good condition without great difficulty, while if
not attended to at once when wear begins to show it
will very rapidly increase, to the great detriment of the
road. In making repairs by this method, the material
is commonly placed a little at a time and compacted by
the traffic. The material used for this purpose should
be the same as that of the road-surface, and not fine
material which would soon reduce to powder under the
loads which come upon it. By careful attention to
minute repairs in this manner a surface may be kept
in good condition until it wears so thin as to require
In case the road be of harder material that will not
so readily combine when a thin coating is added, the
repairs may not be so frequent, as the surface will not
wear so rapidly and immediate attention is not so im-
portant. It is usually more satisfactory in this case to
make more extensive repairs at one time, ?,s a larger
quantity of material added at once may be more
readily compacted to a uniform surface, the repairs
taking the form of an additional layer upon the road.
BROKEN-STONE ROADS. 1 09
Where the material of the road-surface is very hard
and Durable, a well-constructed road may wear quite
evenly and require very little, if anything, in the way
of ordinary small repairs until worn out. It is now
usually considered the best practice to leave such a
road to itself until it wears very thin, and then renew it
by an entirely new layer of broken stone placed in the
same manner as in original construction, on top of the
worn surface, and without in any way disturbing that
If a thin layer only of material is to be added at one
time, in order that it may unite firmly with the upper
layer of the road it is usually necessary to break the
bond of the surface material before placing the new
layer, either by picking it up by hand or, if a steam
roller is in use, by means of short spikes in its surface.
Care should be taken in doing this, however, that only
the surface layer be loosened, and that the solidity of
the body of the road be not disturbed, as might be the
case if the spikes are too long.
FOUNDATIONS FOR PAVEMENTS.
ART. 43. PREPARATION OF ROAD-BED.
IN forming a road-bed upon which to place a pave-
ment, the earth should be brought at subgrade to the
form of the finished road-surface, leaving room for the
superstructure of uniform thickness to be placed upon
it. Thorough drainage must of course be carefully at-
tended to when necessary. This has been already dis-
cussed in Chapter II.
The road-bed after being brought to the proper grade
should be thoroughly compacted by rolling before
placing the pavement. Sometimes in the use of a
heavy roller, when the material is of a light nature, it
is shoved forward in a wave before the roller and re-
fuses to become compacted, in which case a thin layer
of gravel or small stone placed upon the surface of
earth before rolling may have the effect of consolidat-
ing the road-bed under the roller to a hard surface.
If the material over which the pavement is tc be con-
structed is a retentive clay which would become soft
when wet, it is sometimes desirable to excavate the
clay to a depth of 6 inches or I foot below the grade
of the road-bed and fill in with sand or some other
porous and non-retentive material and consolidate this
FOUNDATIONS FOR PAVEMENTS. Ill
to form the road-bed. This would seem unnecessary
in case a sand or gravel foundation is to be used, and
its necessity in any other case would depend upon the
likelihood of the road-bed becoming wet, either through
natural wetness of the soil or in consequence of the
use of a pavement with open joints or of pervious ma-
terial. If the clay substructure can be kept dry it will
sustain the loads which may be carried by the road and
need not be replaced.
In constructing a road-bed to bear a pavement the
same principles would be involved as in the earthwork
of a common road which has been discussed in Art. 25.
ART. 44. PURPOSE OF FOUNDATION.
The chief object of the foundation or base of a
pavement is to distribute the concentrated loads which
come upon the surface of the road over a greater area
of the usually softer and weaker road-bed, in order
that these loads may not produce indentations in the
In a foundation composed of independent blocks
extending through its thickness, as in the case of a
stone-block pavement in which the blocks rest directly
upon the road-bed or upon a thin layer of sand, the
load which comes upon the top of any block will be
distributed over the area covered by the base of the
Where the foundation is composed of small indepen-
dent particles, like sand or loose rounded gravel, with
no cohesion through the mass, the pressure is distrib-
uted over the base of a cone whose vertex is in the
point of application of the load, and the inclination of
112 A TEXT-BOOK ON ROADS AND PAVEMENTS.
whose elements depends upon the friction of the par-
ticles of the material upon each other. In this case
the area over which the load is distributed varies
directly as the square of the thickness of the founda-
tion. Sand, it is to be observed, has also the property,
when confined as in a foundation, on account of its in-
compressible nature, of adjusting itself to a uniform
pressure and resisting the deformation of the road-bed.
If the small pieces composing the foundation are
cemented together, or held as in masses of angular
fragments by the interlocking of the angles, the foun-
dation may act more or less as a whole, causing a distri-
bution of the load over a considerable area, the extent
of which will depend upon the resistance of the mass
The bases most commonly employed for pavements
are sand, broken stone, and concrete. Foundations of
brick and wood are also frequently employed for pave-
ments of the same materials.
ART. 45. SAND FOUNDATIONS.
Sand foundations as sometimes used under block
pavements consist simply of a bed of sand from 6 to
12 inches deep, spread over the road-bed and thor-
oughly compacted by rolling.
To obtain the best results the sand should be placed
in layers of 3 or 4 inches depth and each layer be
rolled before the addition of the next. This insures
the equal consolidation of the entire foundation. Un-
der a heavy roller the sand may usually be compressed
to j or even f of its original volume.
FOUNDATIONS FOR PAVEMENTS. 113
Well-constructed sand foundations are often very
efficient for quite severe service.
Where, as is very common, pavements are set on a
bed of loose sand, subsequent settlement of the base
is likely to take place, even if the surface of the pave-
ment be well rammed during construction.
Block pavements are also frequently set upon a thin
layer of loose sand. This, however, can hardly be
considered a sand foundation, as the sand only acts as
a cushion to protect the earth of the road-bed from
direct contact with the blocks. It also at the same
time may facilitate drainage.
ART. 46. BASES OF GRAVEL AND BROKEN STONE.
Foundations of gravel and broken stone are con-
structed in much the same manner as those of sand.
Small gravel will act in much the same manner as
sand. In general, however, as a base for a pavement
composed of independent blocks these larger materials
are inferior to one either of sand or concrete.
These foundations may also be constructed by the
use of binding material sufficient to thoroughly con-
solidate the mass, after the manner of broken-stone
roads, and when carefully made are quite efficient in
In Liverpool a base has been largely used under
stone-block pavements, which consists of broken stone
grouted with coal pitch and creosote oil, then covered
with quarry-chippings and thoroughly rolled. This is
said to give satisfactory results on streets of moderate
traffic, and is cheaper than hydraulic concrete.
114 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 47. CONCRETE BASES.
The best base for general use under pavements is
without doubt that formed of good hydraulic cement
concrete. A bed of concrete made of good hydraulic
cement, well rammed and allowed to set and harden,
becomes a practically monolithic structure, nearly im-
pervious to water and possessing a high degree of
strength against crushing.
The concrete is formed of a mixture of cement, sand,
and broken stone or gravel. The proportions vary for
different work and with the character of the materials.
With good Portland cement, the most common pro-
portions for ordinary work are about one part cement,
three parts sand, and five to seven parts broken stone.
With the various natural cements the proportions vary
from the above to one part cement, two parts sand,
and three of broken stone.
In preparing the concrete the cement and sand should
first be thoroughly mixed dry, then sufficient water
added to reduce the mass when well worked to a suf-
ficiently plastic condition to be coherent. It should
not be made soft or semi-fluid. The amount of water
necessary to give the proper consistency should be
first determined, and then this quantity added each
time to the mixed sand and cement. The mortar
should then be reduced to a plastic condition by work-
ing it, and not by the addition of more water. The
water should never be applied to the mortar from a
When the mortar has been well mixed it is added to
the broken stone, which should first be dampened by
sprinkling sufficiently to wet the surfaces and wash the
FOUNDATIONS FOR PAVEMENTS. 11$
dust from them. The mass is then to be mixed until
the mortar and stone are uniformly distributed through
it. This is commonly done by turning the whole mass
over several times with shovels.
The concrete, when well mixed, is placed in position
and rammed. The more thorough the ramming the
The foundation, after completion, is allowed to re-
main several days before the pavement is placed upon
it, five or six days are usually sufficient, in order that
the mortar may become entirely set. During setting
the concrete should be protected from the drying ac-
tion of the sun and wind, and should be kept damp to
prevent the formation of drying cracks.
The sand used for mortar should be clean and sharp.
The broken stone should be limited to 2% or 3 inches
in largest dimension, and in some case is limited to 2
Small gravel is sometimes used in place of sand for
the mortar to form concrete, although the smaller ma-
terial is preferable. Gravel is also sometimes mixed
with the broken stone in order that a less quantity of
mortar may be necessary to fill the voids in the stone,
thus making an impervious but weaker concrete.
In some cases foundations are prepared by placing
the broken stone and the mortar upon the road-bed in
alternate layers, and mixing by ramming them into po-
sition. A layer of stone is first placed and wet, then a
thin layer of mortar ; a second layer of stone is added
and rammed into the mortar, after which other lay-
ers are placed and rammed in like manner. This
method has been followed for the best class of work
in Liverpool, and is reported as giving good results.
Il6 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Frequently in laying asphalt pavements, a concrete
is used for the base, which is formed by mixing
asphaltic or coal tar paving cement with the broken
stone. This is known as a bituminous base and is
similar to that mentioned at the end of Art. 46. It is
commonly constructed by placing the broken stone
upon the road-bed to a depth of about 4 or 5 inches
and rolling it to a firm and even bearing, as in the con-
struction of a broken stone road, after which a coating
of coal-tar cement is given to it, about one gallon of
the cement being required for a square yard of the
ART. 48. BRICK FOUNDATIONS.
Foundations of brick have frequently been used
under brick pavements. The pavement in such cases
consists of two layers of brick, with sand between, and
is known as double-layer pavement. These foundations
are usually formed by placing upon the road-bed a
layer of sand or gravel 3 or 4 inches thick, which is
rolled thoroughly to a uniform surface, and then re-
ceives a layer of brick, commonly laid flat and with
the greatest dimension lengthwise of the street. These
bricks are laid as closely as possible with broken joints.
The joints are filled with sand carefully swept in, and
the bricks are rammed to a firm bearing.
Upon this course of brick is placed a cushion layer
of sand, and then the surface layer. The bricks of the
lower layer may be of a cheaper grade than the sur-
face-paving brick, as they are not required to resist the
attrition of travel
FOUNDATIONS FOR PAVEMENTS. 1 1/
ART. 49. SAND AND PLANK FOUNDATIONS.
Under many wood pavements, and sometimes under
brick surfaces, foundations formed of sand and planks
are used. These foundations differ somewhat in
construction in various localities, but are essentially a
bed of sand or gravel, upon which is placed a layer of
tarred boards which support the surface layer.
It is common to use a layer of sand 3 or 4 inches
thick, which is compacted by rolling, after which the
boards are laid lengthwise of the street close together,
so as to form a floor upon which the blocks may be
set. With a brick surface a cushion coat of sand is
used under the surface layer.
Sometimes two layers of one-inch tarred boards are
employed, the lower being laid crosswise of the street
and the upper lengthwise of it. In other cases, the
boards of a single thickness are nailed to scantling laid
across the street and bedded in the sand. The boards
must in all cases press evenly upon the layer of sand
that covers the road-bed.
ART. 50. DEPTH OF FOUNDATION.
The, thickness required for the foundation of a pave-
ment depends upon the nature of the soil upon which
it is to rest, and upon the extent and weight of the
travel to which it is to be subjected.
On a well-drained road-bed of good material, a depth
of 6 inches of concrete is usually sufficient, even under
heavy loads. 6 or 8 inches of well-compacted sand
or gravel will likewise usually suffice.
When, however, the road-bed is of a retentive ma-
Il8 A TEXT-BOOK ON ROADS AND PAVEMENTS.
terial and likely to become wet and soft, the founda-
tion should possess sufficient strength not to be broken
through at points where the supporting power of the
road-bed may be destroyed by water. It must also be
able to resist the action of frost upon the soil below.
In such cases 12 inches of concrete may be necessary,
or 12 to 1 6 inches of sand or gravel may be desirable.
Under light traffic with good conditions, a less depth
may be sometimes used, 4 inches of concrete is fre-
quently employed to save expense, although 6 is the
more common depth.
It is always important that the foundation be suffi-
cient. The yielding of the base of the pavement means
If a firm and durable foundation be employed, the
surface may be renewed when necessary or changed
from one material to another without disturbing the
base, but if the base be weak the surface will be de-
The saving of expense should be at the top rather
than at the bottom of a pavement.
It may be here observed that no definite prescrip-
tion for any pavement, either as to choice of founda-
tion or as to methods of construction can fit all cases.
What is most successful in one case is quite inappli-
cable in another. The blind following of particular
rules by those not conversant with the principles upon
which they are based has been the cause of many fail-
ures. Judgment must always be used in weighing the
local conditions of the problem in hand.
ART. 51. PAVING-BRICK.
THE requisites for a good paving-brick are that it
shall be hard, tough, and impervious, as well as capable
of enduring against the disintegrating influences of the
The bricks in most common use are made from fire-
clay of an inferior quality, or from an indurated clay or
shale of somewhat similar composition.
These clays consist essentially of silicate of alumina,
with usually small percentages of lime, magnesia, iron,
potash, soda, and sometimes other elements. The
range of composition for clays in common use is ap-
proximately as follows:
Silica 60 to 75 per cent.
, Alumina 10 to 25 " "
Iron oxide 3 to 8 " "
Lime o to 4 " "
Magnesia o to 3 " "
Potash 0.5 to 3 " "
Soda - o to 2 " "
In a few cases the quantity of lime is greater vary-
ing from 8 to 12 per cent.
120 A TEXT- BOOK ON ROADS AND PAVEMENTS.
When the clay is very nearly pure silicate of alumina,
it is capable of withstanding a high degree of heat
without fusing, and is known as fire-clay. As the per-
centages of other ingredients increase it becomes more
fusible. The lime, magnesia, potash, and soda act as
fluxing agents, and the readiness with which the clay
can be melted depends upon the relative quantities of
refractory and fluxing materials that it may contain.
Silica in excess tends to make the brick weak and
brittle, while too great quantity of alumina causes the
brick to crack and warp in the shrinking which occurs
during burning. The proper adjustment of the rela-
tions between these elements is necessary to good
The quantity of lime in the clay is an important
matter, as the presence of lime in an uncombined state
in the brick may be productive of disintegration when
the brick is exposed to the weather. A large percent-
age of lime in a clay is therefore to be regarded with
suspicion, although not necessarily as cause for con-
demnation, as its effect depends upon the state of com-
bination of the various ingredients of the brick. Mag-
nesia probably acts in much the same manner as lime.
Potash and soda are considered desirable elements in
quantities to properly flux the clay in burning.
The fineness of a clay is also a matter of importance,
both because a fine clay will fuse at a lower tempera-
ture than a coarse one, and because fineness is neces-
sary to the production of even and close grained brick,
and therefore conduces to make them tough and im-
To produce a good paving-brick, a clay is required
which will vitrify at a high heat. A very refractory
BRICK PAVEMENTS. 121
clay will make a porous brick, while if it melts at too
low a temperature it cannot be burned sufficiently to
become hard and tough.
The methods of manufacturing paving-brick vary in
different localities according to the character of the
material to be worked. They are quite similar to those
in use for common brick, only more thoroughly exe-
The clay is commonly reduced to a fine powder,
tempered with water and passed through a machine
that moulds the bricks, which are then dried and after-
ward burned. Repressed bricks are those which are
compressed in a mould after coming from the brick
machine and before drying.
The process of burning occupies usually from 10 to
The heat is at first slowly applied to expel the water,
then raised to a high temperature for several days,
after which the bricks are very slowly cooled.
There is considerable difference of opinion among
engineers and manufacturers as to the exact amount of
burning necessary. It is usually stated that the brick
should be burned to the point of vitrification, but not
completely vitrified. The burning must be thoroughly
done to produce a strong and impervious brick, but
there is undoubtedly a point beyond which, for some
brick, further burning causes brittleness. Very gradual
cooling is also necessary in order to toughen the brick.
Smoothness and uniformity of texture in a paving
brick is an important consideration as affecting its re-
sistance both to crushing and to abrasion. The broken
surface of the brick should present a uniform appear-
ance both in texture and in color.
122 A TEXT-BOOK ON ROADS AND PAVEMENTS.
All of the bricks used in the same pavement should
also be of the same degree of hardness and toughness
in order that the pavement may wear evenly, and to
this end careful inspection should always be given to
the bricks proposed for use, and all of those which are
defective, soft from imperfect burning, brittle from
everburning or quick cooling, or cracked, should be
The usual and most convenient size for the paving
bricks is about the same as that of building bricks.
These dimensions, however, vary considerably in prac-
tice, and scarcely any two manufacturers make them
exactly of the same size. The ordinary dimensions are
from 7 to 9 inches long, 2 to 2j inches wide, and 3^
to 4^ inches deep, requiring from 50 to 75 bricks per
square yard when set on edge. The corners are some-
times rounded off by a curve of J to ^ inch radius, or
bevelled off, which is not an advantage on ordinary
work where close joints are desirable. There seems to be
no advantage to be gained in making larger bricks, as has
been proposed. It is difficult to burn a larger brick, a
better foothold is given by the pavement with joints
close together, and if a firm foundation be secured the
4-inch depth is ample, while the numerous joints intro-
duce no element of weakness.
ART. 52. TESTS FOR PAVING BRICK.
To determine the probable durability of brick de-
signed for use in paving, mechanical tests may be
applied which will show the relative rank of different
samples in their most important characteristics. The
tests ordinarily proposed or used for this purpose are
BRICK PAVEMENTS. 12$
those of crushing strength, transverse strength, abra-
sion and impact, absorption, and specific gravity.
For the crushing tests it is common to use 2-inch
cubes, sawed from the brick, and brought to a surface
by grinding, without cutting with chisels or hammer-
ing the specimens, in order to prevent any injury to the
material which might reduce its power of resistance.
A sheet of soft paper or a thin layer of plaster of Paris
is sometimes used between the ends of the specimen
and the compression blocks of the testing-machine to
equalize the pressure over the surface of the cube.
The result of a compressive test of stone or brick
depends very largely upon how it is made, and the
results of tests are only properly comparable with
others made in the same manner and with equal care.
The use of plaster beds as suggested above, it is
thought, conduces greatly to regularity of result in the
work of different men, as it tends to reduce the effect
of differences in the accuracy of dressing the surfaces
of contact. The size of the test-piece is also important,
the strength usually increasing as the size increases.
Small pieces, ij or 2 inch cubes, are usually employed,
because of the large force necessary to crush a whole
brick, although where machinery exists capable of doing
it the larger tests entail much less work in preparing
It is to be observed that the actual crushing strength
of a brick is not a matter of special importance in
so far as any danger of the crushing of the material
in the pavement is concerned, as no stress can there
come upon it under ordinary circumstances which
would endanger even a very weak specimen from direct
crushing. It is thought, however, that to some extent
124 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the value of the brick is indicated by its resistance to
crushing, coupled, of course, with a proper examina-
tion of its other necessary attributes. A brick which
possesses a high crushing strength is not necessarily
a good paving brick, as it may at the same time be
brittle or of such composition as to easily disintegrate
under the action of the weather; but one that yields to
a low crushing strength is usually weak in wearing
qualities and not fit for the purpose. A goc d paving
brick, in the form of a 2-inch cube, will usually show a
resistance to crushing of not less than 10,000 pounds
per square inch. Much higher values are sometimes
used in specifications, but their advantage is at least
The transverse strength is tested upon a whole brick
by supporting it on edge upon knife-edges 6 inches apart,
and bringing the load by a third knife-edge upon the
middle of the brick. Soft paper, cloth, or leather may
be interposed between the knife-edges and the brick to
prevent the abrasion of the brick at the points of con-
tact. This test is easier to conduct satisfactorily, and
probably gives, in general, a more reliable indication of
the value of the material than the crushing strength.
It calls into play not only the compressive but the ten-
sile strength of the brick. In the conduct of the test
<:are is, however, quite as essential as in the crushing
test. It is especially important that the bric shall
have a perfectly even bearing upon the supports before
the application of the load, in order that it may not be
subjected to a twist under the load.
The modulus of rupture for the material may be
deduced by the ordinary formula R = r- t in which
BRICK PAVEMENTS. 125
R is the modulus of rupture in pounds per square
inch, W\s the breaking load at the centre in pounds,
/ is the length, b the breadth, and d the depth of the
specimen, all in inches.
The modulus of rupture of paving bricks of good
quality ranges from 1500 to 3000 pounds per square
The fracture of a tough and homogeneous specimen
under a transverse load should be a clean break
through the middle of the brick, and a close observa-
tion of the breaks may frequently be of considerable
assistance in forming an idea of these qualities,
although they may not be directly represented by the
load required to break the specimen. The shattering
of the brick in breaking, or irregular breaks extending
from the point of application of the load to one of the
points of support, are apt to indicate brittleness of the
Tests for abrasion and impact have been conducted
in various ways, and there is no standard method by
which the value of the material to resist these forces
may be quantitatively expressed. Each set of experi-
ments usually compares the various specimens by sub-
jecting them all to the same treatment at once, and in-
cluding, as a standard, specimens of stone or brick
of known value.
The, usual method of conducting this test is to place
whole bricks in a foundry tumbler with a given weight
of cast-iron, and determine the loss of weight of the
specimens after a certain number of revolutions. It is
customary to repeat this operation for two or three in-
dependent periods, usually about one half hour each.
The loss during the first period is largely influenced by
126 A TEXT-BOOK ON ROADS AND PAVEMENTS.
the chipping of corners of the bricks, and the test of
wear would be based more upon the later periods. The
results of such tests made at different times and places
are not comparable with each other, but they are useful
as showing the comparative merit of the samples at
hand in each case.
Where specifications include a test of this character,
it is better to require that the brick shall bear a certain
relation to a standard material to be included in the
test rather than to name a minimum percentage of loss,
as there is necessarily considerable uncertainty in the
test, unless it can be conducted according to a method
the results of which upon standard material are definitely
known, and the details of which may at any time be
reproduced both as to the apparatus to be used and as
to the nature, sizes and forms of the abrading material.
It would be advantageous if certain standards could be
recognized and used in such work which would enable
comparisons to be made of different material upon the
basis of percentages obtained by various men in differ-
It is also desirable to use pieces less in size than a
whole brick, in order that the abrasion during the test
may not be altogether upon the outside of the brick.
It is quite true that the action to which the material
is subjected in a test of this character is quite different
from the wear to which the material is subjected when
firmly held in the pavement, but the qualities necessary
to resist wear in the two cases are quite similar. We
may form an idea of whether a material is suitable for
the proposed use from such experiments, although no
definite idea of the amount of wear that it will endure
can be obtained from them.
BRICK PAVEMENTS. I2/
Absorption tests are made by weighing the specimen
dry, then saturating it and weighing again, and stating
the absorption as a percentage of the dry weight. In
making the test, the brick is first thoroughly dried by
placing it in a drying oven at a temperature of 212
Fahr. until it ceases to lose in weight. It is then placed
in water and permitted to remain until saturated and
weighed again. In some cases the brick is left in the
water twenty-four hours, in others until it ceases to
materially gain in weight. The latter is preferable, as
the 'absorption of various bricks may differ not only in
amount, but also in rate. Whole bricks should not be
used for absorption tests, as the outside is likely to be
less absorptive than the interior. A good paving-brick
will not usually absorb more than 5 per cent of water,
and many of the best varieties will take less than I per
Tests for the presence of free lime may be made by
placing a specimen in water and leaving for a few days.
If uncombined lime be present in considerable quantity
it will cause the brick to crack or blow on the surface.
Tests for this purpose may also be made by pulverizing
a small portion of the brick, washing it with water, and
determining the percentage of soluble matter contained
by the brick.
Such tests may aid in forming a judgment as to the
value of a material for paving purposes, the only con-
clusive test, however, is the record of use of the same
material in similar work. If the normal value of a cer-
tain make of brick be known, tests may indicate
whether a special lot be of standard quality. They
cannot be conclusive as to the value of an untried
128 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 53. FOUNDATIONS FOR BRICK PAVEMENTS.
A brick pavement should have a firm foundation.
As the surface is made up of small independent blocks,
each brick must be adequately supported from below,
or the loads coming upon it may force it downward
and cause unevenness. The wear of the pavement
depends very largely upon the maintenance of a smooth
even surface, as any unevenness will cause the bricks to
chip on the edges, and also produce impact from the
loads passing over the pavement.
The best foundation for a brick pavement is doubtless
one of concrete, laid after the manner given in Art. 47.
For light or moderately heavy traffic, such as that of
the ordinary small city, the concrete is usually placed
6 inches thick. If the traffic be very heavy 9 inches
may be necessary, and where from any cause the road-
bed is not firm it may be advisable to still farther in-
crease the depth.
Under comparatively light traffic a foundation of
gravel or broken stone as mentioned in Art. 46 may be
used. This foundation should, however, usually be
employed only where traffic is light and the road-bed
The double-layer pavement (see Fig. 22) consists of
a foundation made by placing a layer of sand or gravel
3 to 5 inches thick upon the road-bed, rolling it
thoroughly and laying a course of bricks upon it. The
bricks are laid flat with their greatest dimension length-
wise of the street, as explained in Art. 48.
This foundation has been more extensively used
under brick pavements than any other, and has often
given satisfactory results. It is now largely giving
BRICK PAVEMENTS. I2Q
place to concrete in the better class of work, and in
many cases under light traffic its economy is question-
able, as the layer of gravel would often answer equally
well without the lower layer of bricks. The best base
to use for a particular work must usually be largely
determined by the availability of various materials.
ART. 54. CONSTRUCTION OF BRICK PAVEMENT.
In laying a brick pavement, after the completion of
the foundation as described in Chap. VI and Art. 53,
a cushion coat of sand is spread over the surface of the
foundation, f inch to i inches thick, to receive the
surface layer of bricks. The cushion coat should be
composed of clear sharp sand, and quite dry when the
surface brick are placed upon it. It is also desirable
that the sand layer be rolled with a light roller and
brought carefully to surface, in order that it may afford
firm and even bearing for the brick.
The brick surface should be composed of carefully
selected material, as durable, impervious, and uniform
as possible. The bricks should be laid on edge, in
close contact with each other. They are usually arranged
in courses at right angles to the line of the street, the
greatest length of the brick being across the street, and
the bricks in adjoining courses breaking joints with
each other. The laying is begun at the curb, alternate
courses beginning with whole and half bricks and work-
ing to the centre, the work at the curb being carried
but very little ahead of that at the middle of the
street, in order that partial courses may not be dis-
turbed before being completed across the street. This
system of construction is shown in Fig. 21, which repre-
I3O A TEXT-BOOK ON ROADS AND PAVEMENTS.
sents a pavement as constructed for heavy traffic on
heavy concrete foundation.
In many cases the gutter-bricks are turned with the
greatest dimension lengthwise of the street, with the
object of facilitating the flow of surface-water in the
gutter. The advantage of this is doubtful, as it has
the effect of breaking the bond of the pavement be-
tween the gutter-bricks and roadway. This is shown
in Fig. 22, which shows the construction of a double-
layer pavement with brick and gravel base, as has been
commonly used under light or moderate traffic.
Some engineers advocate laying the brick at an angle
BRICK PAVEMENTS. 13!
of 45 degrees with the street line, except on streets
where there are street-railway tracks. Mr. Niles Merri-
wether, City Engineer of Memphis, Tenn., in his report
for 1893, expresses the opinion, based upon experience
in that city, that this arrangement of bricks conduces
to good wear in the pavement.
After the surface layer of brick is in position it
should be rammed or rolled to a smooth and uniform
surface. Usually a heavy wooden rammer is employed
for this purpose, and the ramming should be so
thoroughly done as to discover any weak places that
may exist in the pavement by forcing the bricks down
out of surface at such points. When such places are
discovered the bricks should be removed and the sand
filled in below to properly support them.
When the bricks are well rammed and brought to
the proper surface the joints should be filled with
material of an impervious nature that will cement the
bricks together and form them into a solid and imper-
vious roadway. Various mixtures of coal-tar and
asphalt are commonly used for this purpose. A grout-
ing of hydraulic cement mortar is also sometimes
employed for this purpose.
After the joints are well filled a coating of sand J or
J- inch thick is given to the pavement and it is opened
for trafrlc. In some cases the entire surface of the
pavement is coated with the tar, and the layer of sand
is applied hot, with the view of insuring the binding of
the surface-bricks and rendering the pavement less
In many cases also the tar filling is omitted altogether
and the surface after ramming is covered with the sand
layer and opened to travel.
132 A TEXT-BOOK ON ROADS AND PAVEMENTS.
There is considerable variation in the methods of
construction employed in different localities, as to the
kind of foundation used, the arrangement of the surface-
bricks, and the filling of the joints. A single-course
pavement on a light gravel foundation with joints filled
only with sand or small gravel has frequently been
used for light traffic under favorable conditions with
satisfactory results. Solid and impervious construction,
however, will always give the best results in wear, and
usually will be most economical in the end.
ART. 55. MAINTENANCE OF BRICK PAVEMENTS.
The maintenance necessary for a brick pavement
consists in keeping it clean and carefully watching it,
especially during the first year or two years, to see that
no breaks occur due to the use of defective bricks in
the surface or to insufficient support from the founda-
tion at any point. When any unevenness from either
of these causes appears, it should be at once rectified
before the pavement becomes irregularly worn in con-
While, as already stated, the utmost care should
always be taken to use only material of a uniform
quality in the surface of the pavement, still under the
closest inspection some inferior material may be used,
which will only be shown when wear comes on the
pavement, and unless then removed at once it will
cause the evenness of the surface to be impaired about
it. Irregular support from the foundation will be
less likely to occur in good construction, but its effect
will be similar to defective material, the sinking of in-
dividual bricks producing uneven wear. Weak spots
BRICK PAVEMENTS. 133
in the foundation may sometimes be caused, where con-
crete foundation is not employed, by surface-water
which is permitted to pass through the joints, saturat-
ing the sand or gravel beneath and causing it to move
under concentrated loads. For this reason the joints
should be observed during the early wear of the pave-
ment in order to remedy any case where they may not
have been properly filled.
Where a brick pavement has been constructed of
good material and kept in good surface during the
early period of use, it may then reasonably be expected
to wear out without any considerable expense for small
repairs. The length of time the pavement may be ex-
pected to wear depends upon the quality of the ma-
terial and the methods of construction. For the
moderate traffic of many of the smaller cities, and
lesser streets in the large cities, brick has shown an
endurance which indicates it to be a satisfactory and
economical material, and it is not improbable that by
careful attention to proper construction it may be used
for even heavier traffic.
ART. 56. ASPHALTUM.
ASPHALTUM is a mineral pitch which occurs in
a number of localities widely distributed over the
surface of the earth. It is supposed by most authori-
ties to be the result of the decomposition of vegetable
matter, although by some it is considered to be of vol-
canic origin. It consists, in its natural state, of bitumen
with a small percentage of foreign organic matter and
mixed with more or less mineral earth, and varies
according to the nature of the bitumen from the soft
viscid condition of mineral tar to a hard brittle sub-
stance of glassy appearance and conchoidal fracture.
It also occurs in a solid state as a rock impregnated
with bitumen, which will be separately considered in
another article under the head of rock asphalt.
The asphaltum used for street pavements in this
country is obtained for the most part from the island
of Trinidad, W. I., and from the state of Bermudez,
Venezuela. This asphaltum is known as lake asphalt
or as land asphalt according to the source from which
it is derived. Lake aspJialt is found in large deposits
known as the pitch lakes. The lakes cover a consider-
able areaj in Trinidad about 100 acres ; in Bermudez
ASPHALT PAVEMENTS. 135
several hundred, and the pitch seems to be, or have
been, forced upward from below through fissures in the
rock or craters. The pitch upon contact with the air
gives off gas and gradually hardens. In the lakes
proper the asphaltum is more or less in motion, and
excavations in the surface are soon filled by the flow
of material from the sides and bottom. The pitch near
the centre of the lake at Trinidad is more soft than
near the sides, and it has 'been supposed that the
supply from subterranean sources still continues to
some extent. It has also been found that the surface
of the lake is higher in the centre than at the sides,
and that the general elevation of the surface has been
lowered somewhat by the large quantities of material
which have been removed from it.
The so-called land asphalt from Trinidad is found in
vicinity of the lake, and is a harder material than the
lake asphalt, probably from longer exposure to the air.
It may have been derived either from overflow of the
lake or from independent subterranean sources the
action in which has now ceased. (For a complete
description of the Trinidad pitch deposits see the
" Report of the Inspector of Asphalts and Cements of
the District of Columbia " for 1891-92.)
Asphaltum similar in character to that already men-
tioned occurs at many other places. Mines are worked
in Cuba and Peru, and large deposits are found in
Mexico. It occurs at a number of places in Europe,
while the bituminous mortar of the ancient Chaldean
constructions was of this character, and beds of as-
phaltum are still found in that country from which it
may have been obtained. In the United States de-
posit? of asphaltum are found in California, Utah,
136 A TEXT-BOOK ON ROADS AND PAVEMENTS.
West Virginia, and other places. Those of California
have to some extent been applied to paving purposes,
but have not as yet been largely employed.
The crude asphaltum usually contains considerable
water as well as earthy and vegetable impurities. It is
heated in a boiler to 300 or 400 Fahr., the water
being driven off, and the impurities settling to the
bottom or forming a scum on top. The liquid as-
phaltum is then drawn off and is known as refined
asphalt. This refined asphalt may contain more or
less mineral or earthy matter distributed through it in
a finely divided state. Refined Trinidad asphalt may
contain 52 to 56 per cent of pure bitumen, while Ber-
mudez asphalt is said to contain 97 per cent.
The refined asphalt is brittle at ordinary tempera-
tures, and possesses little cementitious value. To re-
duce it to a condition from which it may be easily
compacted in the pavement, it is heated to a tempera-
ture of about 300 Fahr. and mixed with the oil re-
siduum obtained from the distillation of petroleum.
This mixture is very thoroughly worked, so as to form
a material of uniform consistency. The product is then
known as asphaltic paving cement.
Great care is necessary in mixing the paving cement
to properly proportion the ingredients, as the value of
the cement depends upon their nice adjustment. Both
materials are quite variable in their properties, and
continual tests are necessary of the materials employed
as well as of the resulting product, in order to obtain
a cement of uniformly good quality. The quantity of
oil necessary varies with the nature of the asphaltum,
being less as the asphaltum is of a more plastic and
less refractory nature.
ASPHALT PAVEMENTS. 137
The surface material for a pavement of asphaltnm is
formed by mixing asphaltic paving cement, prepared
as already described, with sand, or with sand and pow-
dered limestone. The proportions of the various in-
gredients depend upon their character and upon the
requirements of the pavement. In order to obtain
good results it is necessary that the exact nature of
each of the ingredients be determined, and the proper
amounts used. With Trinidad asphaltum the surface
material ordinarily contains about 10 per cent of pure
The sand used for this purpose is usually very fine,
60 per cent being sometimes required to pass a sieve
of 60 meshes per linear inch, and all of it through one
of 30 meshes. It is important also that the sand be
clean and free from loam or clay. The carbonate of
lime is used in the form of fine powder.
In forming the surface material the sand and paving
cement are separately heated to a temperature of
about 300 Fahr. and mixed, while hot, in an apparatus
which thoroughly incorporates them into the mixture.
When powdered limestone is used it is added cold to
the hot sand before mixing with the hot paving
As may be readily seen, the selection of asphaltum
for paving purposes, as well as the process of forming
the surface material, is a matter requiring very great
care and an intimate knowledge of the characteristics
of the materials to be employed. In order to secure
good results it is quite essential that careful examina-
tion be made of every mixing of the material for the
surface, both by testing the materials before they are
used and the product after it is formed. Analysis of
138 A TEXT-BOOK ON ROADS AND PAVEMENTS.
every lot should be made as well as consistency tests.
Experience is the only guide, and there is no method
of judging of value in such work other than by the re-
sults of former work of the same kind.
Variations in the quality of asphaltum are attributed
to the character as well as the quantity of bitumen
that it contains. This bitumen is made up of
two parts, the first of which, called petrolene, is the
oily and cementitious material ; the other, called
asphaltene, is the hard material lacking, in the ce-
menting properties. The varying proportions of these
ingredients in the bitumen determine the temperature
at which it will melt, and the facility with which it may
be used for paving purposes. A certain proportion of
asphaltene is probably necessary to give the material
sufficient stiffness at ordinary temperatures; but too
large a quantity makes it lacking in plasticity and
cementing properties, and renders it necessary to use
a large proportion of residuum oil in preparing the
The residuum oil also varies considerably iivcharacter,
and the quantity to be used in each case depends
somewhat upon the character of the oil. The object
aimed at is to get a cement of given consistency, which
is measured by the penetration of a needle, at a stand-
ard temperature and under a standard weight. In
forming the paving cement it is necessary that the
materials be constantly agitated in order that the oil
and asphaltum may be intimately mixed, and that the
mass of cement may be uniform throughout.
For making a paving cement, in some cases where
American asphalt has been used, a maltha, or liquid
bitumen, has been substituted for the residuum oil to
ASPHALT PAVEMENTS. 139
reduce the asphaltum to plasticity. This maltha is a
nearly pure bitumen, similar in character to the asphal-
tum, but differing in that the bitumen contains a high
percentage of the hydrocarbon known as petrolene
and but very little asphaltene, the proportion between
the two being such as make the bitumen liquid at
The composition of surface material for asphalt
pavements must be varied to suit the conditions under
which each is built. The variations of temperature
to which the pavement may be subjected are of special
importance, and the nature of the traffic may also have
an influence. The surface must not soften under the
heat of summer, and yet must be sufficiently plastic
not to become brittle and chip off in cold weather.
For light traffic the material may be more soft in warm
weather than under heavy traffic, as it is not so liable
to cutting under the loads.
The quantity of paving cement used in the surface
material may depend somewhat upon the consistency
adopted for the cement. If it be stiff a larger per-
centage may be used, with the same effect as to the
softening of the surface in summer than if it be soft.
The common proportions of ingredients employed in
forming the surface material are approximately as fol-
For Trinidad Asphalt. For Bermudez Asphalt.
Asphaltic cement 12 to 15 per cent. 9 to 10 per cent
Sand 83 to 70 " 71 to 60
Carbonate of lime 5 to 15 " 20 to 30 "
In some cases stone-dust, formed by crushing a hard
stone, such as granite, to about the dimensions of a
I4O A TEXT-BOOK ON ROADS AND PAVEMENTS.
fine sand, is substituted for a portion of the sand and
carbonate of lime.
The proper treatment in any instance can only be
determined by a careful study of the materials to be
used, the climatic conditions, and the service required.
ART. 57. ROCK ASPHALT.
Rock asphalt, as commonly used in paving, consists
of limestone naturally impregnated with bitumen in
such proportion as to form a material which may be
softened by the action of heat and again consolidate
when cooled if brought under pressure.
This rock is mined at several places in Europe,
notably at Seyssel, France ; Travers, Switzerland ;
Ragusa, Sicily, and Vorwohle, Germany. It is usually
composed of nearly pure carbonate of lime, impreg-
nated with from 7 to 20 per cent of bitumen. It
occurs in veins, after the manner of coal, is hard at the
ordinary temperatures of the mines, and is quarried by
the use of explosives.
The preparation of the surface material of rock
asphalt consists only in crushing and grinding the
rock to powder and heating the powder to drive off the
water and soften it, so that it may be compacted in
the roadway. The powder is heated to a temperature
of from 200 to 300 Fahr., and is applied while hot in
laying the surface.
Natural rock asphalt, suitable for paving purposes,
usually contains from 9 to 12 per cent of bitumen. If
it contain much more than this it is apt to become
soft in warm weather. If it contain less it will not
consolidate properly or bind well in the pavement.
ASPHALT PAVEMENTS. 14!
The rock should be of fine even grain, and have the
bitumen uniformly distributed through it. In form-
ing the surface material for rock asphalt pavements,
the rock from different mines is commonly mixed in
such proportions as to give about 10 or 12 per cent of
bitumen to the mixture, thus making a harder surface
than would be obtained by the use of the rich rock
alone, as well as less likelihood of softening. No other
material is mixed with the rock in forming the
In determining a mixture of asphalt rock, as in the use
of the lake asphalt, the local conditions of climate and
traffic must be considered, and the quantity of bitumen
be so proportioned as to remain solid in summer and
not become brittle and lose cohesion in winter. Ex-
perience with the material and exercise of great care
in the determination of proper proportions is therefore
essential to success in the construction of any asphalt
In the use of bituminous limestone for sidewalks
and many other purposes where a plastic material is
required, the rock asphalt powder is mixed with an
additional quantity of bitumen, or asphaltum, sufficient
commonly to give a product containing 15 to 1 8 per
cent of bitumen. This product is known as asphalt
mastic in Europe. For use in sidewalks the mastic is
melted and mixed with sand or gravel to form a wear-
A sandstone impregnated with bitumen occurs at a
number of localities in the United States. This stone
has been applied to some extent in paving, in some-
what the same manner as the European material. It
142 A TEXT-BOOK ON ROADS AND PAVEMENTS.
is still in the experimental stage, and has not come into
ART. 58. ASPHALT BLOCKS.
Asphalt paving blocks are commonly formed of a
mixture of asphaltic cement with crushed limestone.
This limestone is crushed to sizes of inch in diameter
or less, and mixed with the asphaltic paving cement
formed as described in Art. 56, in such proportions
as that the product contains about 10 per cent of
The materials are heated to a temperature of about
300 Fahr., and mixed while hot in an apparatus
arranged to secure the even distribution of the ingre-
dients through the mass. The thorough incorporation
of the various materials in the mixture is of first im-
portance in producing homogeneous and uniform
blocks, while the quality of the materials used needs
as careful inspection as in the case of the surface mate-
rial for sheet pavements.
When the mixing is complete, the material is placed
in moulds and subjected to heavy pressure, after which
the blocks are cooled suddenly by plunging into cold
These blocks have usually been made larger than
paving-bricks, the common size being 12 inches long,
4 inches wide, and 5 inches deep. They are laid in the
same manner as brick, as closely in contact as possible,
and driven together. Under the action of the sun and
the traffic, the asphalt blocks soon become cemented
together, through the medium of the asphaltic cement,
and form, like the sheet asphalt pavements, a practi-
cally imperivous surface.
ASPHALT PAVEMENTS. 143
They are commonly used on streets of light traffic
only, as the blocks as heretofore constituted wear
rapidly under heavy traffic. They are usually laid
upon a foundation of sand or of sand and gravel, and
on account of the impervious nature of the surface
may often give satisfactory results on such a founda-
tion, where a more pervious block pavement or a sheet
pavement would require more efficient support.
A large amount of pavement, of blocks made prac-
tically as described above, has been laid in this country,
particularly at Baltimore and Washington, and have
shown good durability in wear under moderate and
These blocks have the advantage over sheet asphalt
for the smaller cities, that the blocks may be formed
at a central-point and shipped ready for use to the site
of the proposed pavement, and that no special plant
need be erected in each town where they are to be
In forming the asphalt block pavement the road-bed
is brought to subgrade in the ordinary manner and
rolled, leaving room for the pavement of uniform thick-
ness to be placed upon it. A layer of gravel 4 or 5
inches deep is then placed and rolled, with a cushion
coat of sand I to 2 inches, and then the paving blocks.
The blocks are pressed together in the courses by the
use of a lever, and the courses driven against each
other with a maul to reduce the joints as much as
possible. A coating of sand is given to the surface of
the pavement, and it is rammed to a firm and uniform
surface, as in the case of brick.
144 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 59. FOUNDATIONS FOR SHEET PAVEMENTS.
As a sheet asphalt surface has no power to sustain
loads, acting only as a wearing surface, which must be
held in place from below, it is essential that it be
placed upon a very firm, unyielding foundation. It is
consequently nearly always placed upon a concrete
base, which is commonly formed of hydraulic cement
mortar and broken stone, prepared as described in
Art. 47. In the use of this base, it is necessary that
the mortar be fully set, and the concrete thoroughly
dry before the asphalt is laid upon it, as the placing of
the hot surface material upon a damp foundation will
cause the blistering and possible disintegration of the
surface by the steam generated from the base by the
heat of the material.
For moderate or heavy traffic in cities, the concrete
base is commonly made 6 inches thick. For lighter
traffic a less depth, 4 inches or 5 inches, is sometimes
employed. The depth necessary will depend upon the
nature of the road-bed as well as the weight of the
traffic. It should be greater as the subsoil is less firm
and well drained.
Frequently the concrete for the foundation is formed
of asphaltic or coal tar paving cement instead of
hydraulic cement mortar. It is then known as bitu-
minous concrete, and the foundation is called a bitumi-
nous base to distinguish it from the ordinary hydraulic
base. The advantage claimed for the bituminous base
is that the foundation and surface material become
joined into a single mass, with the effect of anchoring
the surface and preventing the formation of weather-
cracks and wave-surfaces, which sometimes occur when
ASPHALT PAVEMENTS. 145
the hydraulic base is used, under a light surface layer,
in consequence of the lack of bond between the hy-
draulic concrete and the asphalt surface. Another
reason for the use of this base is that it is cheaper than
the hydraulic base.
An intermediate layer known as the binder course
is now commonly added to the foundation, or rather
placed between the foundation and surface layers.
This layer is usually formed of coal-tar or asphaltic
paving cement, mixed with small broken stone, not
more than I inch in diameter, about one gallon of the
cement being required to I cubic foot of stone.
The materials are mixed hot and laid and rolled in
the same manner as the surface layer. This binder
becomes consolidated with, and gives added depth and
strength to, the surface ; thus having a tendency to pre-
vent the cracks and wave-surfaces which may other-
wise appear in the surface when used upon an hydraulic
The hydraulic base is usually preferred to the bi-
tuminous base on account of its forming an unyield-
ing structure, not likely to be forced out of place by
the weight of the traffic at any point where the support
of the road-bed may be weakened, while the bituminous
concrete has not the strength to resist deformation un-
der heavy loads unless uniformly supported. The join-
ing of the base and surface into one mass, as is effected
by the bituminous base is also a disadvantage when the
pavement is to be resurfaced, as with the hydraulic
concrete base the surface may be easily stripped off,
and a new surface placed without injury to the founda-
146 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 60. CONSTRUCTION OF SHEET PAVEMENTS.
The construction of asphalt pavements in this
country is, in the main, in the hands of two or three
large corporations, and methods of construction vary
but little, the differences in the various pavements being
principally due to differences in the composition of the
In constructing the pavement after the completion
of the foundation as indicated in the preceding article,
the surface material is brought to the place where it is
to be used in a large kettle and applied in a hot, semi-
plastic condition and thoroughly consolidated by roll-
ing. The tools with which the material is handled are
kept hot, hot rakes being ordinarily employed for
spreading it, and hot rollers for the first compacting.
Sometimes, as already indicated, a binder course is
inserted between the base and surface layers. This
binder course is usually ij inches thick, composed of
coal-tar cement and small broken stone, and applied in
the same manner as the surface layer. This construc-
tion has been considerably used in Washington, and a
pavement so constructed is known as the combination
In some cases where the binder course is omitted,
the surface is applied in two layers, of which the lower,
known as the cushion coat, is made from inch to
I inch thick when compacted, and contains a higher
percentage of paving cement than the surface layer.
This layer having more of the cementitious material
adheres more strongly to the hydraulic base, and forms
a tie between foundation and surface.
When a bituminous base is employed the cushion
ASPHALT PAVEMENTS. 147
coat is generally considered unnecessary, as the surface
layer will join directly with the base. The binder
course is, however, frequently used in this case to give
added strength and weight to the pavement.
The surface coat, prepared as described in Arts. 56
and 57, is usually applied so as to be about I \ to 2^
inches thick when compacted in the pavement. When
a binder course is employed about I j- inches, otherwise
2 inches to 2j inches.
In the construction of rock asphalt pavements, which
have been almost exclusively used in Europe, it is com-
mon to ram the surface layer with hot rammers, and
smooth it off with smoothing-irons, while in this country
small hot rollers have usually been employed. The
practice of the rock asphalt companies has also differed
from that of those using lake asphalt, in that the latter
continue rolling the surface with heavy rollers until it
is hardened and shows no mark, while the European
practice is to roll more lightly and leave the final com-
pression to be given by the traffic.
In Europe foundations of hydraulic cement concrete
are exclusively used, with the surface layer usually
directly in contact with it. This system is also most
commonly used in this contry, the standard pavement
being formed of an hydraulic base 6 inches thick with
a single surface layer 2\ inches thick when compacted,
or where lighter construction is admissible, an hydrau-
lic base 4 inches thick is used with a surface layer 2
inches in thickness.
In all asphalt pavements it is customary during the
rolling to give the surface a coating of hydraulic cement,
which is usually swept lightly over the surface.
148 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 61. VULCANITE OR DISTILLATE PAVEMENT.
Numerous attempts have been made to construct a
pavement by the use of coal-tar as a cementing material
.in place of or in conjunction with asphaltum. But few
of these have met with any degree of success. Coal-
tar by itself, as most commonly employed, is soon dis-
integrated by the action of the weather ; it is also
strongly affected by temperature, becoming soft in hot
weather and brittle in cold weather.
Pavements in which the wearing surface is composed
of a mixture of coal-tar and asphaltum have in some
cases given good results in practice. These pavements
are known as vulcanite or coal-tar distillate pavements.
They are much cheaper than asphalt. They are said
to be somewhat less slippery, and to resist better where
exposed to dampness. The vulcanite surface, however,
is not so durable under wear, and it requires very great
care in construction to produce a surface of uniformly
good quality, because of the possible variations in the
nature of the coal-tar used.
In the preparation of the surface material for distil-
late pavement as it has been used in Washington, a
paving cement is used containing 70 to 75 per cent of
coal-tar paving cement and 25 to 30 parts refined as-
phaltum. The construction of the pavement is in other
respects similar to that of the combination asphalt
pavement on a bituminous base. As commonly con-
structed the pavement includes a 4-inch bituminous
base, a i-inch binder-course, and a surface of tar dis-
tillate ij- inches thick. The material for the wearing
surface for this pavement, according to the specifica-
tions of 1892, was formed as follows:
ASPHALT PAVEMENTS. 149
Clean sharp sand 63 to 58 per cent.
Broken stone or rock dust. .23 to 28 " "
Paving cement 13 to 15 " "
Hydraulic cement 0.9 " "
Slaked lime 0.15 " "
Flour of sulphur o.i " "
The materials are heated to 250 Fahr. and mixed
hot, then laid after the manner of asphalt.
ART. 62. MAINTENANCE OF ASPHALT PAVEMENTS.
To give good service asphalt pavements must be
kept clean. On account of the smooth surface and
absence of joints, cleaning may be readily accomplished ;
and the presence of dirt, especially in wet weather
when it is likely to cause the surface to remain damp, is
liable to cause the asphalt to rot. More than any
other pavement, therefore, the durability and wear of
an asphalt surface depends upon its cleanliness. The
presence of dirt upon asphalt in damp weather is also
important in its effect upon the slipperiness of the
Small repairs of any breaks that may occur in an
asphalt surface may be easily made, and such repairs
should be constantly attended to in order to keep the
surface in good condition. Small breaks will rapidly
extend if they are not repaired at once. In making
repairs to the surface of the pavement it is necessary to
cut away the surface for a short distance about the im-
perfect spot, stripping the surface from the foundation
and cutting the layer down square at the edges, after
which a new piece of surface may be introduced to fill
the hole in the same manner that the original surface
I5O A TEXT-BOOK ON ROADS AND PAVEMENTS.
was constructed. Such a patch may ordinarily be put
on so as to make joints that will join perfectly with the
old pavement and not show where it has been placed.
When a surface has become so worn that patches would
be numerous, the old surface may be stripped off and
a new one placed upon the original foundation. When
repairs are to be made upon a pavement having a bitu-
minous base it is more difficult to cut out the holes in
satisfactory shape, as there is no well-defined joint be-
tween the base and the surface layers.
The repairs that may be required upon an asphalt
pavement depend, of course, upon the solidity of con-
struction and the nature of the surface material. There
is so great variation in the materials employed for the
wearing surface that, as would naturally be expected,
very considerable difference in wear is shown by dif-
ferent pavements. Asphalt pavements for the most
part, as has been stated, are built by corporations em-
ploying corps of expert workmen, and the plan usually
adopted is to require the contractor to keep the pave-
ment in repair for a certain length of time without
charge and afterwards to maintain it for a certain longer
period at a fixed annual price. This makes it an object
for the contractor to do good work, and is the mos
effective way of securing it where so many elements of
ART. 63. WOOD BLOCKS.
WOOD for pavements should be close-grained and
not too hard. It should be as homogeneous as pos-
sible in order that the wear may be uniform, and soft
enough that it may not wear smooth and slippery. To
give good service in wear the wood should be pene-
trated by water as little as possible and show good
resistance to decay under the action of the weather.
Wood for this use should be sound and well seasoned.
The blocks should always be subjected to careful in-
spection. All sapwood needs to be removed in order
to lessen the liability to early decay, and blocks con-
taining shakes and knots should be rejected.
In the United States cedar has been most largely
used for this purpose and has proved to be a quite
satisfactory wood for such use. Yellow pine and tam-
arack have also been employed for pavements to some
extent at the North, and cypress, juniper, cottonwood,
and mesquite at the South. These varieties have all
been used with some success, and can be made to fur-
nish a fairly good paving material.
Oak and other hard woods are less likely to wear
evenly in the pavement, become smooth ancl slippery,
and bear less well the exposure to the weather and in-
152 A TEXT-BOOK ON ROADS AND PAVEMENTS.
fluences tending to cause decay in the wood. In Wash-
ington, D. C., a pavement of hemlock blocks was at one
time constructed on quite an extensive scale, but proved
unsatisfactory and was soon destroyed. It does not
appear, however, to have been a well-constructed pave-
ment or of properly selected material.
In Australia hard-wood blocks have been quite ex-
tensively used and are reported as giving good service,
although they are admitted to be somewhat slippery in
wet weather. Australian Karri and Jarrah woods are
employed, and it is claimed for them that they show
unusually great resistance to wear and are not soon
affected by decay.
In London, where wood pavements have been very
extensively employed, Swedish yellow deal is com-
monly placed at the head of the list of woods in value,
yellow pine and Baltic fir being also largely used and
considered good in use. The Australian woods above
mentioned have also been used to some extent in Lon-
don, and are said to have given very satisfactory ser-
vice, showing greater resistance to wear than deal or
pine, although somewhat expensive.
Wood pavements are commonly constructed of blocks
set with the fibres vertical, so that wear comes upon the
ends of the fibres and has no tendency to split pieces
off from the blocks. Cedar blocks are used in the form
of whole sections of the tree, on account of the liability
of the wood to split off between the layers when cut to
a rectangular shape. They are usually of an approxi-
mately cylindrical form, varying from 4 to 9 inches in
diameter and 5 to 8 inches in depth. Commonly the
whole section is used with the bark removed, giving
blocks somewhat irregular in shape. In some cases the
WOOD PAVEMENTS. 153
blocks are, however, cut to a true cylindrical form, the
sapwood as well as the bark being cut away, by passing
them through a set of knives which are gauged to
turn out cylinders of given size. Several sets of knives
are employed, giving blocks of varying sizes, each block
being cut sufficiently to eliminate all sapwood with as
little waste of good material as possible. The use of
sapless blocks increases the life of the pavement by
augmenting the resistance of the material both to wear
of the traffic and to the disintegrating influences of the
atmosphere and moisture.
Wooden paving-blocks other than cedar are usually
of rectangular form, 8 to 12 inches long, 3 to 5 inches
wide, and 5 to 8 inches deep. As the blocks are usu-
ally laid in courses, the width as well as the depth must
be constant for the same work. It is usual to cut the
blocks from plank of uniform thickness, ordinarily 3
inches, as the narrow blocks give a better foothold for
horses in damp weather, and also are more easily
settled to a firm and even bearing in the pavement.
Hexagonal blocks are also sometimes used. In a
mesquite pavement constructed at San Antonio, Texas,
the blocks are hexagonal in form, the tops being
slightly smaller than the bottom, the diameter varying
from 4 to 8 inches, with a depth of 5 inches.
The tendency of recent practice in constructing
wood pavements has been in the direction of making
the depth less than was formerly used. Five inches is
now a very common depth.
Deep blocks are usually a waste of material, as in
most cases not more than 2 or 2\ inches at most can
be worn from the pavement before it is replaced, even
if the traffic be sufficient to wear it out before it rots.
154 A TEXT-BOOK ON ROADS AND PAVEMENTS.
In the use of the Australian hard woods in London
a less depth of block has been tried and found satis-
factory, and a depth of about 3^ inches is recommended
by some of the engineers.
ART. 64. FOUNDATIONS FOR WOOD PAVEMENTS.
The foundation which has been most commonly used
in the United States for wood pavement is that com-
posed of a layer of boards upon sand, as described in
Art. 49. This foundation has been used in a number
of places with fairly good results, and, under light
traffic, where the first cost of the pavement must be
low, its use may sometimes be advantageous. This
form of construction has the disadvantage of being
less firm than the others usually employed, as well as
that of being perishable in its nature.
The life of the pavement is therefore likely to be
less than upon a more firm and durable base, as the
destruction of the surface of the pavement may be
caused by the yielding of the foundation. There have
been instances recorded where a base of this kind has
worn out two or more surface layers, and there are
others in which the failure of the foundation planks is
the beginning of the destruction of the surface. Much
depends upon the locality and the quality of the work.
On the better class of wood-block pavements, a
foundation of concrete is usually employed. This
gives firm support to the blocks, and admits of
even wear upon the surface of the pavement. A dur-
able foundation also has the advantage that when the
surface layer is worn out, the pavement may be resur-
faced without renewing the foundation.
WOOD PAVEMENTS. 155
In using a concrete foundation, a cushion coat of
sand is commonly employed on top of the concrete in
which to bed the blocks in order that they may be
brought to an even surface. Sometimes a thin layer
of cement mortar is used in place of the sand upon the
concrete; and in London some pavements have been
constructed with a thin layer, about inch, of asphalt
mastic over the concrete, the blocks resting upon the
Broken-stone and gravel bases are also frequently
employed under wood blocks, a layer of boards being
usually placed over the broken stone to form an even
bearing for the bases of the blocks, although some-
times the blocks are bedded in a cushion coat of sand,
as with the concrete foundation. In Duluth, a Telford
foundation has been employed under wood, laid in the
same manner as for a broken-stone road. A layer of
gravel was placed upon the Telford pavement and
rolled to a smooth surface, after which the wood blocks
were laid directly upon the gravel. The reason given
for the application of this foundation instead of con-
crete in this instance is that the road-bed is of soft
material, which in many places could not be compacted
by the use of a heavy roller before the placing of the
concrete, while the stones of the Telford foundation
are forced to a firm bearing and give a uniform sup-
port. This difficulty of a soft road-bed has been some-
times met in other places by rolling a thin layer of
gravel or broken stone into the surface of the road-bed,
the forcing of the stones into the soil causing it to be-
come compact and firm, after which the concrete may
be placed in the usual manner.
It is perhaps even more important that the founda-
156 A TEXT-BOOK ON ROADS AND PAVEMENTS.
tion for a wooden pavement be firm and unyielding
than with other kinds of block pavement. Any small
motion due to the flexibility of the base is likely to
split the blocks, and if through the yielding nature of
the foundation some of the blocks are forced out of
surface so that the surface becomes slightly uneven,
the wear will be very greatly increased over what would
be the case if the foundation were firm and immovable.
ART. 65. CONSTRUCTION OF WOOD PAVEMENTS.
In the construction of pavements of cylindrical
blocks, or whole-tree sections, as in the common cedar
pavement of the United States, blocks of varying sizes
are usually employed, being set in contact with each
other, the smaller blocks between the larger ones, in
such a way as to leave the spaces between blocks as
small as possible. With blocks varying from 4 to 8
inches in diameter, this arrangement gives good foot-
hold for horses, and at the same time reduces to a
minimum the wear due to the blows caused by wheels
sinking into the joints where large spaces exist between
The common arrangement of cedar blocks is shown
in Fig. 23, which represents a section of pavement as
ordinarily constructed on a sand and board foundation.
In the use of rectangular blocks, the blocks are set
with their longest dimension transverse to the length
of the street. They are usually arranged in courses
across the street, being placed close together in the
courses, and arranged to break joints in adjoining
courses. Between courses a joint is usually made J to
J- inch in width, for the purpose of affording a foothold
to horses. In the older pavements of this character a
much wider joint was employed, some as much as an
inch in width, with the idea that they were necessary
to secure proper foothold. Experience has shown,
however, that the wide joints are not necessary ; and it
is now commonly agreed that where, as is now com-
mon, the blocks are limited to a width of 3 inches, a
joint inch in width is sufficient for the purpose. As
in the case of round blocks, durability is greater where
the joints are as small as possible, and the liability of
the fibres of the blocks to spread is eliminated.
The tendency in practice is toward a continual dim-
inution of the width of the joint, and some pave-
ments have been constructed in London in which the
courses of blocks are placed in contact with each other.
These are reported to have given good results in ser-
vice, and to be advantageous in increasing the durabil-
ity of the pavement. In some cases, where the pave-
ments are laid with close joints, an expansion joint ^
inch in width is provided for every 30 inches of length,
in order to provide for the swelling of the blocks. In
other cases, however, this is said to have been omitted
without injury to the pavement.
158 A TEXT-BOOK ON ROADS AND PAVEMENTS.
The method of setting rectangular blocks is shown
in Fig. 24, which represents a wood-block pavement on
a concrete foundation, as commonly constructed.
In laying a pavement of this kind, a course of blocks
is first set across the street, and then a strip of wood
of the thickness of the joint is set against the row of
blocks and left until the next course is placed, or
'ss.tS. './sft. \ \ \ \ \ S-lyVm/S^^^^f/// / //> />
sometimes spuds with heads of the thickness of the
joints are driven to the head in the side of each block,
and the next row of blocks are set against the spuds.
Where blocks are set in courses across a street, it is
necessary that allowance be made at the curb or gutter
for the expansion of the blocks. This is usually ac-
complished by leaving an open or sanded joint until
the blocks have done swelling.
The gutter blocks are very commonly turned with
their lengths along the street, and sometimes the course
next the curb is left out temporarily and filled with
sand to provide for swelling.
Various methods are employed for filling the joints
between the blocks. It has been the common practice
in the construction of the cheaper wood pavements in
the United States to fill the joints with sand and gravel,
sometimes with a coating of tar, or in some cases the
WOOD PAVEMENTS. 159
joint is partially filled with tar and then completely
filled with sand or small gravel. The objection to this
method is that it does not make an impervious joint,
which in a wood pavement, and especially with the
cheaper foundations, is a matter of the utmost im-
The best practice seems to be to fill the lower part
of the joint for about one half the depth with coal-tar
paving cement and the upper part with hydraulic cement
mortar. The cement mortar gives a harder wearing
surface than where the entire joint is filled with pitch.
It also protects the pitch from the softening action of
the sun in warm weather. Where very narrow joints
are employed, the greater wear of the tar cement may
not be of so much importance. Some engineers fill the
entire joint with coal-tar cement, while others use the
cement grout alone. The object in all cases should be
to make a road-surface as nearly impervious to water
The coal-tar paving cement used for the purpose of
filling joints is the same as that used for brick pave-
ments. It is usually formed of coal-tar residuum mixed
with creosote oil or with tar, but it varies widely in
character in different places. Sometimes asphalt is
used, or asphalt mixed with coal-oil, but more fre-
quently the name asphalt is incorrectly applied to coal-
tar products, when it is called asphaltic cement. It is,
of course, always applied hot to the pavement.
When the ordinary coal-tar cement filling is employed,
the joints are first filled nearly full of sand or gravel,
which is rounded down with a bar, after which the hot
cement is poured in until the joint is well filled.
When cement grout is used it should consist of a
l6o A TEXT-BOOK ON ROADS AND PAVEMENTS.
rich mortar, I part Portland cement to 2 parts sand, or
I part natural cement to I part sand.
In London, where the wood blocks are set upon a
layer of asphalt, it is customary to fill the lower part of
the joint with melted asphaltic cement and the upper
part with cement grout. This method is not, however,
The method of procedure in constructing a wood
pavement depends upon the kind of foundation em-
ployed. When a concrete base is used, a cushion coat
of sand about one inch in thickness is usually spread
evenly over the concrete. Upon this layer of sand the
blocks are set, close together if round blocks, or in rows
as already described if rectangular blocks are used.
The blocks are then rammed to a firm and even bear-
ing by the use of heavy wooden rammers, the joints
are filled with gravel and paving cement or grout, as
the case may be, a layer of gravel is spread over the
surface, and the pavement is opened for traffic. Most
of the well-known London wood pavements are con-
structed in this manner, the blocks being of fir or deal,
and cut to a rectangular shape, although, as already
stated, some of the later ones are built with close joints
from which the gravel filling is omitted.
Where the sand and board foundation is employed,
the best practice is probably represented by the method
pursued at Chicago, which is approximately as follows :
A layer of sand 3 inches thick is placed upon the road-
bed, which has been compacted by rolling. The sand
is rammed or rolled until well compacted, and the
foundation layer of 2-inch hemlock planks laid length-
wise of the street, resting at their ends and middles
upon stringers I inch by 8 inches bedded firmly in the
WOOD PAVEMENTS. l6l
sand. Upon this foundation the cedar blocks are set
close together, the joints are filled with small gravel
well rammed, and the pavement is then flooded with
hot tar cement so as to fill the interstices in the joints.
A coating of gravel one inch thick is then placed upon
the pavement, and traffic is allowed to come upon it.
In some places these pavements are constructed with-
out the use of the coal-tar cement, the joints being
rammed full of sand and gravel ; in other cases the
blocks are set upon rolled sand and gravel without the
boards, the blocks being rammed into place ; but other-
wise the construction is the same as above.
The method of construction advisable for any partic-
ular work depends always upon the local conditions
and requirements. To make a good wood pavement
there is necessary a solid foundation, blocks of good
material, and impervious joints, and all such work should
be so constructed as to secure these conditions in so far
as available resources will admit. Weak construction
always involves high cost for maintenance, and greater
expense in the end than good construction.
ART. 66. PRESERVATION OF WOOD.
The most serious objection commonly raised to the
use of wood pavements is that wood, being porous, ab-
sorbs moisture readily, and is thus both liable to de-
struction through decay and to become injurious to
health. Various methods have therefore been pro-
posed for rendering the blocks less pervious and more
durable by impregnating them with various solutions
which shall fill the pores and act as preservatives.
The methods which have been principally
known as Burnettizing, Kyanizing, and
162 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Burnettizing consists in immersing the wood in a
solution of chloride of zinc until the pores are filled
with the solution. This is either done by simply im-
mersing and allowing the wood to gradually absorb the
solution, or by forcing the solution into the pores of
the wood under pressure. The first method requires
considerable time : about two days' immersion to each
inch of thickness is usually allowed in order to admit of
the wood becoming saturated with the solution.
In kyanizing a saturated solution of corosive sub-
limate is used, and the timber immersed in the solution
long enough for the pores to become well filled.
Creosoting consists in impregnating the wood with
the oil of tar or creosote. In this process the wood is
first thoroughly dried, usually by heating it in a kiln, and
the hot creosote is then forced in under pressure. The
method of accomplishing this varies in different places.
In order to be effective the process must be thoroughly
carried out and the pores well filled. It is commonly
recommended that from 8 to 12 pounds of creosote
per cubic foot of timber should be forced in, as a mini-
mum requirement for the softer woods, such as are com-
monly used in pavements. Creosote has the property
of destroying the lower forms of animal life, and is
therefore an effective preservative against destruction
through these agencies where they exist. This method
is therefore often employed for the preservation of
timber for subaqueous construction in sea-water.
All of the above processes, when properly applied,
are effective in preventing decay, and therefore in
lengthening the natural life of the wood. They also
render the wood practically impermeable, and thus re-
move the objection to the pavement based upon its
WOOD PAVEMENTS. 163
absorbent nature. They do not, in general, appear to
increase the resistance of the wood to the wear of the
traffic, and in most cases the advantage to be gained
seems so small as to render their economic value for
this purpose at least doubtful.
The economic advantage of using treated blocks is a
question of the relative costs of increasing the expense
of construction by using them or of the additional ex-
pense of more frequent renewals where they would be
necessary without the treatment. The desirability of
the treatment in any particular instance depends to
some extent upon the traffic to which the pavement is
to be subjected as well as upon the character of the
material available for the purpose. Where the traffic
is such as to bring a considerable wear upon the pave-
ment, and sound, well-seasoned blocks are to be had,
there is usually little, if any, advantage in the treated
blocks, as the pavement will ultimately fail by the wear
of the blocks in either case. Experience has shown
that in many cases the untreated wood wears out in
the pavement before decay sets in, and that the appli-
cation of the preservative processes would not prolong
the life of the pavement. This has been the result of
experience in London, where after trying many differ-
ent methods the consensus of opinion is against the
use of preservative processes. It is claimed, however,
by some authorities that creosoted blocks have been
shown in some instances to give greater resistance to
wear than untreated blocks under the same traffic (see
London Engineering for July 29, 1892). This London
traffic is heavy, the material well selected, and the
wear severe. There may frequently be cases, however,
wherewith lighter traffic or with wood of a less durable
164 A TEXT-BOOK ON ROADS AND PAVEMENTS.
nature or less well seasoned the application of preser-
vatives may effect such a lengthening of the life of the
pavement as to make their application economically
In the treatment of the wood it is essential that the
process be very thoroughly applied in order to get
good results. The process most commonly recom-
mended is creosoting, and in order to derive any bene-
fit from the treatment it is necessary that the pores of
the wood be thoroughly filled with the oil. Merely
dipping the blocks in creosote or tar, as is sometimes
done, is more likely to be an injury than a benefit, and
has been found in some cases to be the cause of decay
by closing the pores upon the surface of the block
and inducing an internal dry-rot. It is also essential
to success with creosote that the blocks be thoroughly
dried before injecting the creosote.
ART. 67. MAINTENANCE OF WOOD PAVEMENTS.
The ordinary maintenance of wood pavements, like
that of most other pavements, consists in keeping the
pavement clean and in repairing from time to time any
small breaks that may appear in the surface due to im-
perfect material or to the settling of the foundation.
These repairs would, of course, include the removal of
any defective blocks and the taking up and replacing of
any portion which may settle out of surface through
It is generally agreed that the wear of a wood surface
is improved by giving it an occasional coating of small
gravel, in some cases two or three times a year, and
permitting it to be ground into the surface for a few
WOOD PAVEMENTS. 165
days. It is an advantage also that the surface be kept
sprinkled in warm weather.
When the wood pavement needs renewal or exten-
sive repairs the surface may be relaid as with any other
block pavement: if a permanent foundation be em-
ployed, by stripping the blocks from the foundation
and placing a new surface in the same manner as the
first one ; with a board foundation that also must be
If the pavement is cut through for any cause the
surface may be replaced with the same facility as other
block pavements ; but where a board foundation is
used it is necessary to use care in replacing in order to
secure proper bond with the remainder of the founda-
tion and prevent any subsequent settlement at the
line of cut. In such cases it is also necessary to com-
pact the earth very carefully in replacing it, that there
may be no subsequent settlement.
The cost of keeping a wood pavement in order, as
with any other pavement, depends upon the character
of the work done in construction, the better the pave-
ment the cheaper the maintenance.
ART. 68. HEALTHFULNESS OF WOOD PAVEMENTS.
The use of wood pavement is very often objected to
upon the ground that it is unhealthful and likely to
give rise to disease. This is based upon the fact that
the material of the pavement, being porous and ab-
sorptive of moisture, is likely to become saturated with
organic matter from the foul liquids of the surface
soaking through it. This foul matter must also, on
account of the permeable nature of the material, pass
1 66 A TEXT-BOOK ON ROADS AND PAVEMENTS.
to some extent through the pavement and contami-
nate the foundation and soil beneath as well, especially
where the foundation is a permeable one, as in the case
of boards or sand and gravel. In addition to the
danger due to the permeability of the wood, unhealth-
fulness may also be caused by the liability to decay
of the material.
There is much difference of opinion among author-
ities concerning the extent of the danger to health
offered by ordinary wood-block pavements, some re-
garding the danger as a very serious one and protest-
ing against the use of wood in any case, while others,
although admitting the permeability and perishable
nature of the material, consider its proper use quite a
safe one and the danger as somewhat visionary. Health
statistics of cities using wood pavements to a large
extent, compared with those of cities not using them, do
not indicate anything unfavorable to them ; but it may
properly be said that such statistics can seldom be
compared in such a manner as to give a reliable index,
as there are so many other circumstances which may
affect public health, and the conditions other than the
pavements are rarely the same.
The likelihood of a pavement producing unsanitary
conditions depends very largely upon climatic and local
conditions and upon the construction of the pavement.
The opinions of those observing the matter are there-
fore usually based upon their own local surroundings.
Instances are recorded where blocks of wood after con-
siderable service in pavements have been found to be
but little, if any, affected by the absorption of the street
refuse, and the foundations to be quite unaffected by it ;
and in other instances blocks have been found to be
WOOD PAVEMENTS. l6/
considerably contaminated, and the foundations and
subsoil saturated with filth. In most cases these differ-
ences may be attributed to differences in methods of
construction as well as in material used, and the cause
of subsoil contamination appears usually to be open-
joint construction rather than permeable blocks.
Where good drainage exists and a pavement is con-
structed of sound, well-seasoned blocks with close, im-
pervious joints, so that it cannot get wet at the base,
the danger from saturation and decay is probably
small, and on the score of health such a pavement is
much to be preferred to a stone-block pavement with
open joints. Under the reverse of these conditions a
wood pavement may be a serious menace to health.
In close, damp places, or climates giving the same
conditions, the liability to decay is much greater than
where the pavement is exposed to the sun and air.
ART. 69. STONE FOR PAVEMENTS.
STONE-BLOCK pavements are commonly employed
where the traffic is heavy and a material needed which
will resist well under wear.
Stone for this purpose must possess sufficient hard-
ness to resist the abrasive action of wheels. It must
be tough, in order that it may not be broken by shocks.
It should be impervious to moisture and capable of
resisting the destructive agencies of the atmosphere
and of weather changes.
Experience only can determine the availability of
any particular stone for this use. The stone may be
tested in the same manner as brick, and perhaps some-
thing predicated as to the probability of its wearing
well under traffic ; but the conditions of the use of the
material in the pavement are quite different from those
under which it may be tested, and any tests looking to
a determination of its weathering properties are apt to
Examination of a stone as to its structure, the close-
ness of grain, homogeneity, etc., may assist in forming
an idea of its nature and value for wear. Observations
of any surfaces which may have been exposed for a
considerable time to the weather, either in structures
STONE-BLOCK PAVEMENTS. 169
or in the quarry, will be the most efficient method of
forming an opinion concerning the weathering proper-
ties of the stone. The conditions of use in pavements
are, however, somewhat different from ordinary expos-
ure in structures, on account of the material in the
pavement being subject to the action of water contain-
ing acids and organic substances due to excretal and
refuse matter. A low degree of permeability usually
indicates that a material will not be greatly affected by
these influences and also that the effect of frost will
not be great.
Granite and sandstones are commonly employed for
paving blocks and furnish the best material. Lime-
stones are sometimes used, but have seldom been found
satisfactory. Trap-rock and the harder granites, while
answering well the requirements as to durability and
resistance to wear, are objectionable on account of
their tendency to wear smooth and become slippery
and dangerous to horses. Granite or syenite of a tough
homogeneous nature is probably the best material for
the construction of a durable pavement for heavy
traffic. Granites of a quartzy nature are usually brittle
and do not resist well under the blows of horses' feet or
the impact of vehicles on a rough surface. Those con-
taining a high percentage of felspar are likely to be
affected by atmospheric agencies, while those in which
mica predominates wear rapidly on account of their
Sandstones of a close-grained compact nature often
give very satisfactory results under heavy wear. They
are less hard than granite and wear more rapidly, but
do not become so smooth and slippery, and commonly
form a pavement that is more satisfactory from the
I/O A TEXT-BOOK ON ROADS AND PAVEMENTS.
point of view of the user. Sandstones differ very
widely in character, their value depending chiefly upon
the nature of the cementing material which holds them
together. In order that a stone may wear well and
evenly in a pavement it is desirable that it be fine-
grained, dense, and homogeneous, as well as cemented
by a material which is not brittle and is nearly imper-
vious to moisture. Those sandstones in which the
cementing material is of an argillaceous or calcareous
nature are apt to be perishable when exposed to the
weather. The Medina sandstones of Western New
York and Ohio have been quite extensively used for
paving purposes and prove a very satisfactory material
for such use.
Limestone has not usually been successful in use for
the construction of block pavements on account of its
lack of durability against atmospheric influences. The
action of frost commonly causes weakness and shiver-
ing, which produces uneven and destructive wear under
traffic. There are, however, as wide variations in the
characteristics of limestones as in those of sandstones,
and there may be possible exceptions to the rule that
in general limestone is not a desirable material for
ART. 70. COBBLESTONE PAVEMENTS.
Cobblestones have in the past been quite extensively
used in the construction of street pavements, although
at the present time they have been for the most part
abandoned, excepting where they are used at the sides
of other pavements for gutter construction or some-
times between the rails of a horse-car track. This
STONE-BLOCK PAVEMENTS. 17 1
pavement as ordinarily constructed is a cheap one in
first cost, and it affords a good foothold for horses. It
is not usually a durable pavement as the stones are
easily loosened from their positions, although the
stones themselves may be practically indestructible
and used again and again in reconstructing the surface.
Cobblestone pavements as commonly constructed
are also objectionable because they are permeable to
water and difficult to clean. They therefore collect
and become saturated with the filth of the street
and are very liable to injury from frost. They are
also extremely rough and unsatisfactory in use for
For paving the side-gutters, where broken-stone or
sometimes where wood is used for the travelled portion
of the street, cobblestones may often be convenient
and useful, and form a cheap and satisfactory means of
disposing of surface drainage. Such an arrangement
is shown in Fig. 31 (p. 186).
Cobble pavements may also sometimes be advanta-
geously used upon steep grades where traffic is neces-
sarily slow and the foothold afforded becomes a very
important matter. When used for this purpose a con-
crete foundation should be employed and the stones
be firmly bedded to prevent displacement through the
efforts of horses to obtain foothold.
Cobblestones as used for pavements are usually
rounded pebbles from 3 to 8 inches in diameter. They
are set on end in a layer of sand or gravel, rammed into
place until firmly held in position, and then covered
with sand or fine gravel and left to the action of travel,
which soon works the upper layer of sand into the in-
terstices between the stones.
172 A TEXT-BOOK ON ROADS AND PAVEMENTS.
ART. 71. BELGIAN BLOCKS.
Belgian block is the name commonly applied to a
pavement formed of nearly cubical blocks of hard rock.
In the vicinity of New York this pavement has been
largely used, the material being trap-rock from the
valley of the lower Hudson. The blocks are usually
from 5 to 7 inches upon the edges, with nearly parallel
faces, and as commonly laid are placed upon a founda-
tion layer of sand or gravel about 6 inches thick. This
shape of block is objectionable on account of the width
between joints being too great to afford good foothold
to horses. The materials of which Belgian blocks have
ordinarily been formed are very hard and (as already
noted in Art. 69) wear smooth in service, becoming
slippery and thus increasing the effect of the too wide
block. It is also better to have the length of the
blocks somewhat greater across the street and let
them break joints in that direction in order that they
may give greater resistance to displacement under pass-
The older pavements of this character were usually
placed upon a sand foundation. More recently, this
practice has, in the better class of work, been super-
seded by a more solid construction, a concrete base
ART. 72. GRANITE AND SANDSTONE BLOCKS.
For the construction of the better class of stone-
block pavements, blocks of tough granite or sandstone
are used, set, in the best work, upon a concrete base,
although sometimes placed upon a foundation of sand
STONE-BLOCK PAVEMENTS. 173
These pavements when well constructed are about
the most satisfactory means yet devised for providing
for very heavy traffic, as they present a maximum
resistance to wear with a fairly good foothold for
horses, and are much more agreeable in service than the
old form of rough pavements. There is still much to
be desired in the attainment of smoothness and ab-
sence of noise, and, as a general thing, it may be said
that pavements of this kind are desirable only where
the weight of traffic is so great that the smoother pave-
ments would not offer sufficient resistance to wear.
Even in such cases it may frequently be questionable
whether an additional expense for maintaining a pave-
ment which would be more pleasant in use and less
objectionable to occupants of adjoining premises
would not be advisable from an economical as well as
from an aesthetic point of view.
Blocks for stone pavements, in the best work, are cut
in the form of parallelepipeds, 9 to 12 inches long, 3
inches wide, and 6 or 7 inches deep. The length should
be sufficient to permit the blocks to break joints across
the street. The width should be less than that of a
horse's hoof in order that the joints in the direction of
travel may be close enough together to prevent a horse
from slipping in getting a foothold. The depth should
be sufficient to give a bearing surface in the joints
large enough to prevent the blocks from tipping when
the load comes upon one end of it.
ART. 73. CONSTRUCTION OF STONE-BLOCK
Stone-block pavement for durable and ff.2
service should be placed upon very firm foundations.
174 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Bases of concrete are usually employed and give the
best results. These foundations are formed as de-
scribed in Art. 47, and consist of a layer of concrete 4 to
8 inches thick, 6 inches being the most common depth.
In constructing the pavement, a cushion coat of sand,
usually about an inch thick, is spread upon the base of
concrete, for the purpose of allowing the bases of the
paving blocks to be firmly bedded when the tops are
brought to an even surface, the sand readily adjusting
itself so as to fill all the spaces beneath the blocks and
to offer a uniform resistance to downward motion in
every part of the pavement, and in like manner trans-
mitting the loads which come upon the pavement to
the foundation so as to evenly distribute them over the
surface of the concrete. The sand used for this pur-
pose should be clean and dry, and all large particles
sifted out, as they may prevent the blocks adjusting
themselves properly. A thin layer of asphaltic cement
is sometimes used in place of the sand with very good
The blocks should be laid as close together as possi-
ble in order to make the joints small. They are laid,
like brick, with the longest dimension across the street,
and arranged in courses transverse to the street, with
the stone in consecutive courses breaking joints, In
laying, it is considered best to begin the courses at the
gutters and work toward the middle, the crown-stone
being required to fit in tight.
After the blocks are placed they are well rammed to
a firm unyielding bearing and an even surface. Stones
that sink too low under the ramming must be taken
out and raised by putting more sand underneath.
As in the case of other block pavements, those of
STONE-BLOCK PAVEMENTS. 17$
stone should be made as impervious to moisture as
possible. The foundation should be kept dry and
moisture prevented from penetrating beneath the
blocks, where it has a tendency to cause unequal settle-
ment under loads, or disruptions under the action of
frost. In the better class of work, therefore, the joints
are filled with an impervious material which cements
the blocks together. Coal-tar paving cement is com-
monly employed for this purpose, as with brick and
wood, and seems the most satisfactory in use, although
hydraulic cement mortar is sometimes used. The coal-
tar cement is commonly made by mixing coal-tar pitch
with gas-tar and oil of creosote, a proportion sometimes
employed being 100 pounds pitch, 4 galls, tar, and I
The use of cement between the blocks binds them
together and increases the strength of the pavement as
well as the resistance of the blocks to being forced out
of surface. It also deadens to some extent the noise
from the passing of vehicles where asphaltic or coal-tar
cement is used.
The method ^of filling the joints is usually to first
fill them about one third full of small gravel, then
pour in the paving cement until it stands above the
gravel ; then another third full of gravel, more cement
as before ; then gravel to a little below the top, and the
joint filled full of cement ; after which a coating of
fine gravel is distributed over the surface.
Various modifications of the method above outlined
are used in the principal cities for a pavement to with-
stand heaviest traffic and secure a maximum of dura-
bility : essentially it represents the best modern practice.
A cheaper form of stone-block pavement is made by
176 A TEXT-BOOK ON ROADS AND PAVEMENTS.
laying the blocks directly upon a foundation of gravel
or sand, either with cemented joints or with joints filled
with gravel only. This gives a fairly good pavement
for streets of moderate traffic, and has been extensively
used in the past. The present tendency, however,
which will probably increase in the future, is to lessen the
use of pavements of this character, and to substitute a
surface which is more pleasant in use for all service
where durability and resistance to wear are not the
ART. 74. STONE TRACKWAYS.
In some of the European cities, particularly in Italy,
stone trackways are sometimes employed on streets of
heavy traffic for the purpose of diminishing traction.
These trackways are formed of smooth blocks of stone
4 to 6 feet long, 1 8 to 24 inches wide, and 6 to 8 inches
deep, laid flat and end to end so as to form a smooth
surface upon which wheels may move with the least
possible resistance. Between the tracks, and usually
the remainder of the street, is commonly paved with
cobble. The method of construction is shown in Fig.
25. The tracks drain to the middle, and the pavement
between is made concave and provided with openings
into the storm sewers for the escape of surface-water.
The track and pavement are laid upon a layer of sand
resting upon a broken-stone or gravel foundation.
STONE-BLOCK PAVEMENTS. 1/7
Such trackways are very durable under heavy traffic,
and give very light traction combined with good foot-
hold. It is possible that they might advantageously
be applied oftener than they are on streets used for
ART. 75. ARRANGEMENT OF CITY STREETS.
THE location of streets should be planned with a
view to giving direct and easy communication between
all parts of a city. The arrangement should also be
such as to permit the subdivision of the area traversed
by them in such a manner as to give the maximum of
efficiency for business or residential purposes. The
most obvious and satisfactory method of accomplishing
these purposes is usually by the use of the rectangular
system, with occasional diagonal streets along lines
likely to be in the direction of considerable travel.
Streets so far as possible should be systematically
arranged and continuous throughout the extent of the
city, both to facilitate travel and to admit of their being
so named and numbered that the locality of a place
of business or residence may at once be evident, from
its address, to any one familiar with the general plan
of the city. The rectangular system is desirable on
this account, and also because it furnishes blocks of the
best form for subdivision into building lots.
The proper arrangement of streets will always neces-
sarily depend in some measure upon the natural feat-
ures of the locality, and any system of arrangement
will be more or less modified by local topography.
Where for topographic or aesthetic reasons it may be
considered desirable to use curved lines for the streets,
the continuity and uniformity of arrangement should
be maintained as far as possible. The use of curves
on residence streets may sometimes be advantageous
in reducing gradients or in its effect upon adjoining
property through avoiding heavy earthwork. Where
a change in direction is necessary the use of a curve
usually gives a better appearance than an abrupt bend,
unless the change can be effected at the intersection of
a cross-street. Care is required, however, to prevent
the local introduction of curvature disarranging the
general plans and producing the chaotic condition due
to an irregular use of short streets.
In laying out a rectangular system of streets the
blocks ordinarily will preferably be long and narrow.
The distance needed between streets in one direction
is only that necessary to the proper depth of lots, while
in the other direction the streets need only be close
enough to provide convenient communication for the
travel and traffic. A convenient method would be to
lay out the main streets so as to form squares large
enough to permit the introduction of an intermediate
minor street through the blocks. These minor streets
may then be introduced in the direction that seems
advisable in each locality. Such an arrangement is
shown in Fig. 26. The diagonal streets cut more space
from the blocks traversed by them, but give more
frontage, and property fronting them will usually have
more value than other property in its vicinity.
The proper location for diagonal streets intended as
thoroughfares for traffic is naturally determined by the
positions of the business centres or public buildings
ISO A TEXT-BOOK ON ROADS AND PAVEMENTS.
and parks, from which they may radiate in such manner
as to bring the outlying portions of the city into the
most direct communication possible.
A city cannot usually be laid out complete. Its for-
mation is a matter of gradual growth and enlargement,
and the end cannot be seen from the beginning. For
this reason it is frequently necessary to undergo great
expense in the larger cities in cutting new streets or in
changing the positions or dimensions of existing old
ones in built-up districts in order to relieve the
crowded condition of the streets, which hampers busi-
ness and renders travel difficult and unpleasant. Much
of this difficulty might frequently be obviated if in
growing towns and cities proper attention were given
to the regulation of suburban development. Such de-
velopment should be under municipal control so far
as to require at least that each new subdivision which
opens new streets should be made with a view to
affording proper ways of communication between ad-
joining properties by making streets continuous.
Where such regulation does not exist streets will be
1 82 A TEXT-BOOK ON ROADS AND PAVEMENTS.
laid in any manner to best develop the particular prop-
erty in which they are placed.
A good example of the advantages of systematic and
liberal plans in street arrangement, as well as of the
evils of unregulated extension, is given by the case of
Washington, D. C.
Fig. 27 shows a portion of the city of Washington
illustrating its systematic arrangement. It consists of
a rectangular system, together with two sets of diag-
onal avenues, and open squares or circles at the inter-
sections of the avenues.
Fig. 28 shows a number of suburban subdivisions on
the borders of the city of Washington, made previous
to the adoption of the law regulating them. In some
cases the streets of adjoining subdivisions have no
communication with each other, and the general ten-
dency is toward a labyrinth of short streets. The law
now requires that all street extension within the Dis-
CITY STREETS. 183
trict of Columbia shall conform to the general plan of
the city of Washington ; and under the operation of
this law the lines of many of the city streets have been
extended to all parts of the District, and all of the
suburban development is being gradually brought with
the city into one harmonious whole, on the same gen-
erous plan that exists within the city. The rectification
of the irregular plats upon the borders of the city must,
however, be a matter of heavy expense to the District.
ART. 76. WIDTH AND CROSS-SECTION.
The width of city streets is important both on ac-
count of its influence upon the ease with which traffic
may be conducted, and because of its effect upon the
health and comfort of the people, by determining the
amount of light and air which may 'penetrate into
thickly built-up districts.
To properly accommodate the traffic of commercial
thoroughfares in business districts of towns of consider-
able size, a street should have a width of 100 to 160
feet, the whole of it to be used for roadway and side-
walks. Wide streets are especially needed where, as in
the larger cities, they are bordered by high buildings
or are to carry lines of street railway.
Residence streets in a town of considerable size,
where houses are set out to the property line and stand
close together, should have a width of at least 80 to 100
feet in order to look well and give plenty of light and
The streets in nearly all large towns are laid out too
narrow ; they are crowded and dingy. The chief diffi-
culty is that the future of a street is not usually fore-
1 84 A TEXT-BOOK ON ROADS AND PAVEMENTS.
seen when it is located. Owners in subdividing prop-
erty are only anxious to get as many lots as possible
out of it, and there are usually no regulations looking
to the future health and comfort of resident when the
street shall be built upon. In the growth of a town the
nature of localities change : residence streets become
business streets, streets devoted to retail trade become
wholesale streets, and mercantile districts are given up
to manufacturing. If a city could be laid out com-
plete from the beginning it would be comparatively
easy to consider the requirements to be met and locate
the streets accordingly. Under existing conditions
this is not possible, but a more liberal policy in planning
streets would usually be found of advantage in any
growth that may ensue. There is also very frequently
an immediate financial advantage in the enhancement
of values due to wide streets. A lot 100 feet deep on
a street 80 feet wide will nearly always be of greater
value than if the same lot be no feet deep and the
street only 60 feet in width.
In Washington, D. C, which probably has the best
general system of any American city, no new street can
be located less than 90 feet in width, and avenues
must be at least 120 feet wide. Intermediate streets,
called places, 60 feet wide, are allowed within blocks,
but full-width streets must be located not more than
600 feet apart. The value of this liberal policy to the
city of Washington is evident not only in the increased
comfort of the people, but in its large growth as a resi-
dential city and the increased value of property in it.
While it is advantageous to have the street wide be-
tween building-lines, it is not necessary that the whole
street width be used for pavements. The street pave-
CITY STREETS. 185
ment should be gauged in width by the immediate
necessities of the traffic which is to pass over it. The
pavement should be wide enough to easily accommo-
date the traffic, but any unnecessary width is a tax
upon the community in the construction and mainte-
nance of more pavement than should be required, and
perhaps diminishes the length of street which may be
improved with available funds. Thus, for a residence
street in general a width of 30 to 35 feet between
curbs is usually ample, with a foot-walk upon each side
6 to 10 feet wide. The remainder of the street width
should be made into lawns upon each side, with tree
spaces between the sidewalk and roadway.
Fig. 29 shows in partial section the arrangement of a
- - - - - --vL.3,*- -6FV..-*-- UAWN 17 Fh
QO-ft. residence street for moderate traffic. For resi-
dence streets of lesser importance, where the travel
is light and the street is only required to furnish
facilities to meet the needs of its immediate locality, a
less width of pavement may often be advantageously
used. A pavement 24 feet wide is sufficient to accom-
modate a very considerable amount of light driving,
and in many places, especially in the smaller towns
where funds for effective improvement are obtained
with difficulty, even less widths may be employed with
the result of improving the streets both in appearance
and usefulness. All that is really needed in such cases
is room for teams to pass comfortably and to turn
without difficulty. The narrowing of roadways on
streets of light traffic to what is really
1 86 A TEXT-BOOK ON ROADS AND PAVEMENTS.
often make possible improvements which will turn a
broad sea of mud into a narrow hard roadway and a
grass-plat. Fig. 30 shows the arrangement of a village
street 50 feet wide for light service.
In many cases for village streets, where the traffic is
light and it is essential that the cost of construction be
low, it may be good practice to construct the travelled
portion of the roadway of macadam, wood, or other
pavement, and use cobble gutters at the sides without
curbs. Fig. 31 shows a roadway 30 feet wide, with
macadam middle and cobble gutters. In Saginaw,
Mich., this method has been followed, using either
macadam or wood blocks for the middle portion, and
in the report of City Engineer Roberts for 1893 it is
recommended as economical and efficient.
The cross-section of streets must be arranged with
reference to proper surface drainage. The street is
given a crown at the middle to throw the water into
the gutters, and sidewalks usually have a sufficient in-
clination toward the gutter to cause them to drain over
the curb. The section necessary for street drainage is
discussed in Art. 10. The street is usually made prac-
tically level across, the curbs and sidewalks at the two
sides being given the same elevation. The parking at
CITY STREETS. 1 87
the sides may have a slope between the sidewalk and
the building-line when it is necessary or advantageous.
Sometimes, on streets along a slope, expense may be
saved or adjoining property benefited by placing the
sidewalk at a different elevation from that of the street,
as shown in Fig. 5> or by placing one curb lower than
the other and moving the crown of the road to one
side, as shown in Fig. 32.
ART. 77. STREET GRADES.
The grades of city streets necessarily depend mainly
upon the topography of the site. Wherever possible,
it is desirable that grades be uniform between cross-
In establishing grades for new streets through unim-
proved property, they may usually be laid with refer-
ence only to obtaining the most desirable gradients for
the street within a proper limit of cost. But where
improvements have already been made, and located
with reference to the natural surface of the ground, it
is frequently a matter of extreme difficulty to give a
desirable grade to the streets without injury to adjoin-
ing properties. In such cases it becomes a question of
how far individual interests shall be sacrificed to the
general good. It may be said in this connection that
adjustments to new grades are usually accomplished
much more easily than would be anticipated, and when
accomplished the possession of a desirable grade is of
1 88 A TEXT-BOOK ON ROADS AND PAVEMENTS.
very considerable value to adjoining property. Too
great timidity should not, therefore, be felt in regard
to making necessary changes because of the fear of in-
juring property in the locality.
Where a grade if made continuous between inter-
secting streets would be nearly level, it is frequently
necessary to put a summit in the middle of the block
and give a light gradient downward in each direction
to the cross-streets in order to provide for surface
drainage. The amount of slope necessary to provide
for proper drainage depends upon the character of the
surface and smoothness of the gutter. For a surface
of earth or macadam the slope should not be less than
about I in 100, and for paved streets from I in 200 to
1 in 250.
In some cases it may be possible to give sufficient
slope to gutters to carry off the surface-water by mak-
ing the gutter deeper at the ends than in the middle of
the block without making a summit in the crown of
the street. The curb in such case would be made
level or of uniform gradient.
The smoother forms of pavement are only applicable
to light gradients. Rock asphalt is usually limited to
2 or 2^ per cent grades. Trinidad asphalt maybe used
to grades of about 4 per cent. Brick, if kept clean, is
safe on gradients of about 6 per cent ; wood, on those
a little steeper ; and stone blocks are satisfactory to
about a lo-per-cent gradient.
Pavements on steeper gradients must be made rough
in order to insure a safe foothold to horses. On grades
steeper than 9 or 10 per cent cobblestones are prefera-
ble to rectangular stone blocks, as they give better
foothold, and the speed of travel being necessarily
CITY STREETS. 189
slow the roughness is of less consequence. For such
use it is desirable to have the cobblestones set on a
concrete foundation and the joints filled with paving
cement after the manner of a first-class block pave-
ment, as the wear on a steep slope will be severe. Or-
dinary stone blocks may be laid on steep streets with
wide joints, about an inch, so as to give better foothold
than the common form ; or the corners of the stones
may be bevelled on the upper edges and set in the usual
In a report on the streets of Duluth in 1890, Messrs.
Rudolph Hering and Andrew Rosewater recommend
for steep streets, in addition to the above, that brick
may be used in which the tops are rounded, and that
wood blocks for such use have their upper edges cham-
fered on each side, or if round blocks be used, around
ART. 78. STREET INTERSECTIONS.
At intersections the crown of the roadway pavement
on each street should, if possible, be continuous to the
centre of intersection, in order to prevent vehicles on
one street from being subjected to the jar incident to
passing over the gutter of the other. Where a storm
sewer is available into which the water from the gut-
ters on the upper side can be emptied this is a simple
matter, but where such sewers do not exist it requires
the adoption of some special means of draining the
gutters on the upper side. This may sometimes be
accomplished by a culvert across the street, the gutters
being somewhat depressed at the corners to bring the
channel sufficiently low. In other cases, where the
slope is sufficient, it is more satisfactory to construct
I QO A TEXT-BOOK ON ROADS AND PAVEMENTS.
an underground pipe-drain from the upper corner to
some point in the gutter below the crossing.
Where the rate of grade is such that it is feasible, it
is desirable that the grade of both streets should be
brought to a level at intersections. The top of the curb
at the four corners should be at the same elevation, thus
permitting the continuation of the full section of each
roadway until they intersect. It is also desirable that
the sidewalks at the corners be level ; that is, the points
a a in Fig. 33 should all be placed at the same eleva-
tion, which will make the entire street section, includ-
ing sidewalks, horizontal across the direction of travel
on each street.
On very steep slopes it may not be possible to flatten
out the grade to a level in crossing transverse streets,
and in such cases the elevations require study, and need
to be carefully worked out for each particular case. In
the report of Messrs. Rudolph Hering and Andrew
Rosewater upon the streets of Duluth, it is recom-
mended that in all cases the grade shall be reduced to
3 per cent between the curb-lines of cross-streets, and
CITY STREETS. IQI
the grade of the curb reduced in all cases to 8 per cent
for the width of the sidewalks of intersecting streets.
This is to be considered the maximum allowable rate
of transverse grade, and only to be employed in case of
necessity. If in Fig. 33 the arrow represents the direc-
tion of steep slope, and the street transverse to that
direction has a roadway 40 feet wide with sidewalks
10 feet wide, the above limits would permit the curb
at c to be 1.2 feet lower than that at ^, and admit of
a fall of 0.8 foot in the curb line from a to b and from
c to d. If both streets have the same grade and
width the curb at the lowest corner would be 2.4 feet
lower than at the highest corner.
Sometimes, where the parallel streets in one direction
follow the lines of greatest slope, and the cross-street
are normal to them, the proper grades at intersections
may be arranged by giving the streets along the slope
a section similar to that shown in Fig. 32 throughout its
length, thus permitting the street in the direction of
slope to continue its grade across the intersection
without altering at that point the side slope of the
For a case of maximum slope this would make the
section of the roadway of the cross-street a plane sur-
face sloping uniformly from the upper to the lower
curb, or in Fig. 32 it would transfer the street crown to
the upper curb.
ART. 79. FOOTWAYS.
Footways are not required to bear the heavy loads
which come upon the roadway pavement, but in streets
of considerable travel are subjected to a continual
IQ2 A TEXT-BOOK ON ROADS AND PAVEMENTS.
abrading action, and for good service are required to
be of a material which will resist abrasion well, of so
uniform a texture as to wear evenly, and not hard
enough to become smooth and slippery in use.
A good sidewalk should always present an even sur-
face, and therefore requires a firm foundation to resist
the displacement of the blocks of which it may be com-
posed. It must also be durable under atmospheric
changes, and of material that may be easily cleaned.
The materials commonly employed are gravel, wood,
brick, tar, asphalt, stone, and artificial stone.
Gravel walks are the cheapest of footways where
suitable material is available. They are constructed in
a manner similar to that used for gravel roadways, and
require that the bed of the walk be well drained, and
that it be well compacted by rolling or ramming before
the walk is placed upon it. The best gravel walks are
usually built upon a base of rough stone. This base
may be 6 or 8 inches thick, and forms a solid founda-
tion upon which the gravel surface may be placed and
sustained against settling. Walks constructed in this
manner are frequently used in city parks where the
travel is considerable. On suburban roads, gravel
walks usually consist of a thin surface of gravel laid
upon the earth-bed, and are replaced by some other
surface when a more expensive construction can be
afforded. Gutters are frequently necessary to protect
the walks from the wash of surface-water, which other-
wise very quickly destroys it.
Wood is commonly used for walks in the form of
planks, which are laid on stringers, the planks being
placed perpendicularly to the direction of travel. It is
comparatively short-lived, and requires considerable
CITY STREETS. 193
expenditure for repairs, as the material is perishable
and also wears rapidly.
Brick footway pavements have been extensively used
for many years, and form, when well constructed, a very
durable and satisfactory sidewalk. As commonly con-
structed they consist of ordinary hard-burned bricks
laid flat upon a layer of sand over the earth-bed. For
light travel, pavements so constructed may last well
and give good service ; but they are apt to suon become
uneven through the sinking of the bricks because of in-
In constructing such a pavement the sand layer
should be well compacted by rolling or ramming be-
fore setting the bricks, which should also be rammed
to a firm and even bearing. To give satisfactory re-
sults, a foundation of sand and gravel or broken stone
should be formed 8 or 10 inches in thickness. In
Washington a layer of gravel 4 inches thick and well
compacted is used, with a layer of sand of the same
thickness upon it to receive the surface. In forming
the pavements, the bricks are laid flat and as close as
possible. The joints are filled with sand, usually by
coating the surface with a layer of sand before ram-
ming, and after completion a second coating, which is
allowed to remain a few days after admitting the travel
Care must be used in selecting brick for this purpose
to get only hard-burned brick of uniform quality,
in order that the resistance to wear may be even.
The use of vitrified paving brick, as used for roadway
pavement, would be of advantage on walks subjected
to heavy wear.
The use of a concrete foundation and setting the
194 A TEXT-BOOK ON ROADS AND PAVEMENTS.
brick on edge and in mortar, after the manner of con-
structing a roadway pavement, makes a very durable
sidewalk under heavy travel. It is, however, some-
what expensive, and usually a stone surface would be
preferable where such expense is to be incurred.
Footway pavements of a concrete in which coal-tar
is the binding material have been widely used, but have
not usually been satisfactory in use. As commonly
constructed they wear rapidly and soften, becoming
very disagreeable in hot weather. Some pavements of
this character have, however, shown fairly good service.
Numerous methods have been proposed and tried
for the construction of tar foot-walks, differing from
each other in the materials mixed with the tar to form
the concrete, and in the manipulation of the process.
Ashes mixed with sand and gravel are usually em-
ployed, and sometimes clinkers from an iron foundry.
A somewhat successful pavement of this class has a
small amount of Portland cement mixed with the ashes
and sand used in forming the concrete before the addi-
tion of the tar.
Asphalt footway pavements are formed either of as-
phalt blocks or of a surface of sheet asphalt. Where
blocks are used they are laid in the same manner as
brick upon a foundation of sand or gravel. The
blocks, or tiles as they are commonly called, are usu-
ally made flat, about 8 inches square and 2 to 2^ inches
thick. They are laid with their edges either at right
angles to the street line or at an angle of 45 with
the street line, usually at right angles, on account of
greater ease in laying.
Sheet-asphalt footways are laid in the same manner
as an asphalt street pavement, the pavement, however,
CITY STREETS. 195
being given a less thickness. In Washington, D. C.,
these pavements are made about 3 inches thick, and
constructed upon a bituminous base. Material re-
moved from street pavements in resurfacing is used for
forming the surface material of the footway. Mixtures
of coal-tar and asphalt similar to that used for distillate
pavement, as noted in Art. 61, are also used in foot-
ways, and are commonly spoken of as asphalt.
In the use of rock asphalt for footways, the asphalt
mastic mentioned in Art. 57 is commonly used, mixed
with sand or gravel to give a wearing-surface. The in-
gredients are heated together and applied hot to a
broken-stone or concrete foundation. In Europe hy-
draulic cement concrete is used for the base, as in the
driveways. A layer of 3 or 4 inches of concrete is em-
ployed, with a surface layer of rock asphalt or asphalt
mastic and sand, J to f inch in thickness, for ordinary
Natural stone for foot-walks is ordinarily used in the
form of flagging. Where flagstones of proper size and
good wearing qualities may be readily obtained, this
kind of pavement, if well laid, makes a durable and
satisfactory foot-walk. Flagstones should be set upon
a solid foundation and be firmly bedded so as to
preserve an even surface. They should not be laid, as
is common in many places, directly upon an earth-bed,
but should have a cushion layer of sand or of some
porous material to prevent unequal settling under the
action of frost.
Artificial-stone pavements, when well constructed of
good materials, make the most satisfactory of foot-
ways. They form an even surface, quite agreeable in
service, and are durable and economical where exposed
196 A TEXT-BOOK ON ROADS AND PAVEMENTS.
to considerable travel. Pavements of this kind are
either constructed of blocks of material made at a fac-
tory and carried to the site of the walk, or the stone is
formed in the position in which it is to be used. The
latter plan is more commonly followed and admits of
the use of larger blocks, the size in this case being only
limited by the necessity of providing for changes of
dimension with those of temperature, very large blocks
being liable to crack under such changes.
There are a number of methods of preparing arti-
ficial stone for pavements, many of them patented,
differing to some extent in the composition of the
material or the details of the work. In general the
process consists in placing a layer of concrete 4 to 6
inches thick upon a layer of gravel or other porous
material. A surface of rich mortar or concrete, com-
posed of hydraulic cement with sand or crushed
granite, is given to the pavement, and the surface is
commonly roughened by scratching lines upon it
before it is hardened. As with all other concrete-
work, the pavement needs to be kept damp and pro-
tected from the sun until the mortar is fully set. A
layer of damp sand spread over the surface may be
advantageously employed to protect it for several days
after it is opened to travel.
ART. 80. CURBS AND GUTTERS.
Curbs are usually set in the streets of towns at the
sides of roadway pavements for the purpose of sus-
taining and protecting the sidewalk or tree space, and
of forming the side of the gutter. They are commonly
formed of natural stone, but sometimes also of artificial
stone, clay blocks, or cast iron.
CITY STREETS. 197
The curbs used in different places vary considerably
in form and dimensions. Stone curbs vary from 4 to
12 inches in width and from 8 to 24 inches in depth.
They are usually employed from 3 to 6 feet in length,
and set with close joints.
The depth must be sufficient to admit of their being
firmly bedded, and to prevent overturning into the
gutter. The front of the curb should be hammer-
dressed to a depth greater than its exposure above the
gutter, and the back deep enough to permit the side-
walk pavement to fit close against it where the side-
walk adjoins the curb. The ends of the blocks should
also be dressed to the depth of exposure, and the part
below the ground trimmed off so as to permit the
dressed ends to come in contact when laid.
Granite is usually considered the best material for
curbs, although both sandstones and limestones are
used in many places. In the vicinity of New York the
North River bluestone has proved a good material for
There are various ways of setting the curb. The
object should be to bed it firmly on a solid foundation.
The best method is to place a bed of concrete under it.
This construction is shown in Fig. 34, which repre-
sents the method used in setting granite curb in Wash-
ington, D. C. The curb is held firmly in place by the
concrete foundation, which joins it rigidly to the road-
Where the concrete foundation is not used under
the curb a deeper curbstone is necessary, usually from
1 8 to 24 inches in good work. Curbs are very com-
monly set in the natural ground, the pavement coming
against it on one side ; but it is usually found advan-
198 A TEXT-BOOK ON ROADS AND PAVEMENTS.
tageous to lay them upon a bed of gravel or broken
stone, with gravel filled in the trench about them.
The ordinary method of setting curbs is shown in
The Washington specifications for ordinary work
require that a bed of gravel 4 inches deep be used
under the curb, and that the trench be filled with
gravel placed in layers 3 or 4 inches deep, each layer
being thoroughly rammed before adding the next.
Curbs of artificial stone or concrete are usually
CITY STREETS. 199
formed by mixing the concrete upon the ground and
placing it in the position it is to occupy, using a board
mould, as in constructing artificial-stone foot-walks, to
give it proper shape. By this method of construction
the curb and gutter may be made practically in
one piece^ where a concrete base is used for the
pavement. The concrete for the curb and gutter is
made of smaller materials and with a higher percen-
tage of cement than in preparing the foundation for
the roadway, and is given a surface coating of cement
mortar which is commonly formed of a mixture of Port-
land cement with finely crushed granite.
Specifications for artificial-stone curb in Washington,
D. C, require that the concrete be composed of I
part Portland cement, 2 parts clean sharp sand, and
3 parts clean broken stone not more than I inch in
their largest dimensions. The exposed surface of both
gutter and curb is to be coated i inches thick with a
mortar composed of 3 parts granulated granite (the
fragments being of such size as to pass through a
J-inch screen, and free from dust) and 2 parts cement.
Artificial-stone curbs are sometimes made hollow,
and the interior spaces used as a conduit for pipes or
wires. A variety of forms are used for these cases,
the curb being usually made in blocks at a factory and
set like natural stone, the blocks being commonly
formed in separate parts which maybe fitted together
to form the curb and removed to give access to the
openings. Where the hollow curbs are in one piece,
hand-holes are placed at short intervals to admit of
using the openings ; this may be done in case the con-
duits are to be used for wires.
Curbs of burned clay or brick are made in several
200 A TEXT-BOOK ON ROADS AND PAVEMENTS.
forms, both solid and hollow, and are frequently used
on streets paved with brick, where stone suitable for
curbing is lacking.
Cast-iron curbs are sometimes employed, although
they have not come into use extensively.
They consist usually of a casting similar to
that shown in section in Fig. 36, which forms
the face and top of the curb, being open
at the back and braced with ribs at short
FIG 6 intervals of length. They are held in place
by ties attached to the ribs, and the backs
are filled and tamped to a firm bearing.
Wrought-iron plates or angles are sometimes used as
a protection to concrete, or to resurface a worn stone
curb, the iron being fitted to the face of the curb so as
to form the exposed surface. Several forms are used,
and the process is patented.
Gutters are commonly formed of the same material
as the roadway pavement, which is simply extended
to the curb.
In streets paved with brick or granite blocks the
gutter blocks are sometimes turned lengthwise of the
street, as shown in Fig. 22, for the purpose of facilitat-
ing the flow of water in the gutter. As already pointed
out, however, this has the effect of making a continuous
joint between the pavement and gutter, and its utility
For streets paved with broken stone it is common to
employ stone gutters, formed of cobblestones, of narrow
flags laid lengthwise of the gutter, or sometimes of rec-
tangular blocks. Such construction is shown in Fig. 35.
On streets paved with wood these gutters may also be
frequently employed with advantage, especially where
CITY STREETS. 2OI
for any reason the gutter is likely to be kept damp.
In forming a cobble gutter the stones are usually set
upon a layer of sand or gravel after the manner of
forming a cobble pavement. They should be firmly
bedded and form an even surface.
Cobble gutters are often used on village streets
where no curbs are set, and in such locations where
but slight expense is admissible they are quite satis-
factory if properly constructed. This method of con-
struction is illustrated in Fig. 31.
Sometimes in work of this kind a flagstone is used
for the bottom of the gutter and the sides are formed of
cobble. This is preferable as affording a more free
channel for the flow of the surface drainage.
To obtain satisfactory results it is always necessary
that the foundation be of sufficient depth and well
compacted, in order to prevent the surface becoming
uneven by the stones being forced downward into the
road-bed in wet weather or through the action of frost.
A layer of 6 to 10 inches of gravel or sand is usually
Where flagstones are used to form the gutter, they
should be 3 or 4 inches thick, 10 to 15 inches wide, as
may be required, and about 3 feet long. Care is re-
quired in laying that they may have an even bed and be
well supported by the foundation.
Gutters of bricks, or of stone blocks, are often used for
streets upon which the roadway pavement is asphalt,
on account of the liability of the asphalt being injured
by dampness. In this case the gutter is constructed
by setting the bricks or blocks with their greatest
length along the street. They are placed upon a bed
of concrete, the same as is used for the foundation
202 A TEXT-BOOK ON ROADS AND PAVEMENTS.
under the asphalt surface, and the joints are filled with
coal-tar paving cement, as in constructing brick pave-
It is also advisable in using flagstone gutters that
consecutive blocks should have different widths, differ-
ing by 2 or 3 inches, in order that there may not be a
continuous joint between the flagstones and the pave-
ment of the travelled roadway.
ART. 8 1. CROSSINGS.
On streets paved with a smooth hard surface which
is easily cleaned, such as brick or asphalt, special foot-
way crossings are not usually required or desirable,
unless the foot travel be very considerable. On other
pavements, however, which are apt to be rough to
walk upon or muddy in bad weather, as upon stone,
wood, or macadam, footways of flagstones are com-
monly provided, and form the most satisfactory
These crossings consist of flagstones about 10 or 12
inches wide laid in rows across the street, the rows
being 6 or 8 inches apart and paved between with stone
blocks set in the ordinary manner. The crossing-
stones are 3 or 4 feet long, and at least 6 inches
thick in order that they may not be broken by the
traffic. They should be laid with close joints and
firmly bedded upon the foundation.
At street intersections where the number of pedes-
trians is large it is desirable that the crossing be
carried across on the level of the top of the curb
without leaving a step at the gutter crossing. This
may be accomplished by bridging over the gutter with
CITY STREETS. 203
a flagstone or iron plate, or by placing the outlets for
surface drainage a few feet back from the corner and
eliminating the gutter at the corner.
ART. 82. STREET-RAILWAY TRACK.
Track for street railways upon paved streets should
be constructed with a view to offering as little obstruc-
tion to ordinary street traffic as possible, while per-
mitting the ready operation of the railway. These
two points are apt to conflict, and the interest of the
railway company in the construction of track is rarely
identical with that of the public use of the street.
Track in streets is usually constructed of rails laid
upon cross-ties, either fastened directly to the ties as
in the track of steam roads, or supported upon chairs
which serve to raise the surface of the rail to a greater
height above the tie, and in some cases to hold the
ends firmly at the joints. Sometimes, also, the rails are
laid upon longitudinal wooden stringers placed upon
the ties, or bolted together by iron rods across the
track without the use of ties. Fig. 37 shows this sys-
tern of construction, without the stringers, the rails
being set directly upon the concrete foundation.
Iron ties have been used to a limited extent, and in
some cases the rails are set upon chairs resting upon
204 A TEXT-BOOK ON ROADS AND PAVEMENTS.
The best track from the standpoint of the operation
of the railway is probably that formed of ordinary T
rails laid directly upon the cross-ties without the use
of chairs, in the manner used for steam roads. This
form of construction is, however, usually unsuitable for
track in streets, as the pavement cannot be laid close
against the rail at its upper surface. Where stone or
wood blocks are used with T rails it is necessary to cut
away the corners of the blocks in order to provide a
channel for the wheel-flange. This has a tendency to
induce greater wear under heavy traffic. With brick
pavements bricks are sometimes moulded of special
form, with one corner rounded off, so that they may be
set firmly against the rail and still leave room for the
wheel-flange. This method has proved fairly satis-
factory in some places, but has the disadvantage of
leaving a corner of the brick exposed to wear.
In most cases where T rails are employed, the
rails are allowed to project above the pavement and
form a serious obstruction to the ordinary use of the
street. Even where the track is well constructed and
the pavement originally made even with the top of the
rail, under any considerable traffic the wear of the
pavement near the rail is usually rapid and the rail
soon projects. This is true to a certain extent with
any rail, but more especially with the T form.
The form of rail now commonly used in good con-
struction is that known as the girder rail, either the
ordinary single web-girder rail as shown in Fig. 38, or
the box-girder rail as in Fig. 39.
The advantage these rails possess over the T rail is
that the pavement may be laid against the rail, flush
with its top surface, the channel for the wheel-flange
being provided by the form given
to the head of the rail. The box
girder is sometimes thought to
possess an advantage over the
single - web rail from the fact that
it affords a vertical surface against
which to place the pavement, and
an even support to the paving blocks
at the bottom as well as at the top, so that there is no
tendency for the block to slip under the flange of the
rail. In the use of the single-web rails the space under
the flanges may with advantage be filled with cement
mortar to form a bearing for the paving block as shown
in Fig. 40.
Where the paving surface used is not too thick,
such as brick or asphalt, the track may usually be
206 A TEXT-BOOK ON ROADS AND PAVEMENTS.
constructed by spiking the rails directly to the ties
as in Fig. 40. If a thicker surface is to be used, as
with a stone-block pavement, the rails must be sup-
ported on chairs, unless rails of extra height be used
or longitudinal stringers are placed under the rails.
Girder rails, as to the form of head, are divided into
centre-bearing, side-bearing, and grooved. Of these
the grooved rail of form shown in Fig. 38, #, or
Fig. 40, is the most desirable, considered with reference
to the ordinary street traffic, and when the pavement
is smooth and kept clean is satisfactory in use. It has
been extensively used in Washington, D. C. The
objection to the use of this form of rail is that the
groove is likely to become filled with dirt, and there-
fore requires constant care to keep clean, especially
where the street is not maintained always in good con-
dition. This disadvantage is greater in cold climates
where snow and ice are common during winter. It
is also necessary with this form of rail that the track
be very accurately gauged in width, in order that the
flanges may properly fit the grooves ; and it is desirable,
especially if the rails be supported on chairs, that the
rails be tied together by rods as in Fig. 37.
It has been claimed that more power is required to
move cars upon rails of this pattern, even under favor-
able conditions, than is necessary on others. The ad-
vantage to the street traffic of using these rails is, how-
ever, very considerable. When placed in a smooth
pavement which is made flush with the top surface of
the rail, the track offers no obstruction to the passing
of vehicles over it in any direction, and the inconven-
ience and difficulty of pulling in and out of the track
The grooved rail of form shown in Fig. 38, b, is some-
times employed, and obviates to a certain extent the
difficulties met in operating track of the form just men-
tioned, the groove being widened at the top so that the
wheel-flange may press the dirt out at the sloping side
and also give more room for the flange.
The side-bearing rail as shown in Fig. 38, c, is prob-
ably more generally used than any other. With this
rail the flange extends out on one side to form a channel
for the wheel-flange. It is more easily kept clear than
the grooved form, but wheels of vehicles readily slip
into the channel and leave it with difficulty, although
when properly constructed such track offers no resist-
ances to vehicles crossing it. Fig. 41 shows a block
pavement with track formed of side-bearing girder rails
supported by chairs which are spiked to the cross-ties.
The centre-bearing rail as shown in Fig. 38, d, forms
the best track to operate, because it keeps clear of dirt
and offers little resistance to the car. It is, however,
the most objectionable to the ordinary street traffic, as
it is difficult for wheels to cross it; and its use is not
commonly permitted on streets of considerable traffic.
Many modifications and combinations of these forms
are employed in different localities, and the number of
small variations which may be introduced is practi-
208 A TEXT-BOOK ON ROADS AND PAVEMENTS.
cally endless. In general, however, nearly all of the
rails in common use belong to one of the three classes
In addition to the T rails and girder rails various
other methods of construction are sometimes employed.
The duplex rail is composed of two parts rolled sepa-
rately and fitting together. The two parts break joints,
the object being to eliminate the weakness of the ordi-
Thin strap-rails, or tram-rails as they are commonly
called, made to be laid upon longitudinal stringers of
wood, are used to some extent, but have in the main
been superseded by the girder forms. They consist
simply of a plate of iron with a head raised upon it,
similar in form to those already mentioned, the plate
being laid flat upon the stringer.
The solid construction of track is a matter of im-
portance upon paved streets, because of the difficulty
and expense of getting at the track to make repairs, as
well as because of the disturbance to traffic when the
pavement must be removed for this purpose. The rail-
joints and tie-connections are therefore matters requir-
ing particular attention. Where no chairs are used,
the use of tie-plates to form a bearing for the rail upon
the tie, and to hold it securely in place, is to be recom-
mended, and will greatly aid in forming a rigid track.
There are a number of forms in use which give good
results. They should be arranged to clamp the rail
firmly and present a good bearing upon the tie. When
chairs are used, they, like the tie-plates, should clamp
the rail firmly and give good bearing surface. They
should also be well braced for stiffness against lateral
Joints, in the case of track formed of rails laid di-
rectly upon the ties, or upon wooden stringers, are
usually made by placing a plate or channel-bar upon
each side of the web of the rail ends to be joined and
bolting through. The use of slightly curved channel-
bars fitting against the flanges of the rail, as shown
in Fig. 42, seems to give good results, the spring in the
channels serving to prevent the
loosening of the bolts.
Where chairs are employed
to raise the rails above the
ties, joints are frequently most
satisfactorily made upon long
chairs or bridges reaching
across the space between two
ties and forming a firm bear-
ing for the ends of the rails.
In order to facilitate keeping
the joints tight and enable
the bolts at the rail ends to be screwed up without
taking up the pavement, joint-boxes are sometimes
employed. These consist of openings with removable
covers, giving access to the bolts at the ends of the
It is essential to any good track construction that
the track be well ballasted and be brought to an even
bearing upon the road-bed ; otherwise the track will
spring under passing loads and soon become uneven
and out of surface with the pavement. Gravel or
broken stone is usually preferred for ballast, but where
first-class pavements are employed, founded upon a
concrete base, the track should also be set in
Crete. This practice has been commonly
210 A TEXT-BOOK ON ROADS AND PAVEMENTS.
Europe with good results. The ballast should be
firmly tamped about the ties, which are preferably of
hewn timber on account of the greater ease of tamping.
The wear of a pavement is usually considerably in-
creased by railway tracks upon the street. The extent
of this wear depends upon the nature of the paving
surface as well as upon the construction of the track.
It is mainly the difference in resistance to abrasive
wear between the rails and the paving surface which
causes uneven and more rapid wear of the pavement in
vicinity of the track. A broken-stone surface, on ac-
count of its rapid wear, is particularly objectionable
along a line of track, and is very difficult to keep in
Where T-rail construction is used there is a largely
increased wear due to the exposed edges of the paving
blocks, which wear rapidly on the sides and in the
grooves left for the wheel-flanges. (See article by W.
L. Dickinson in Good Roads for May 1894.) With a
smooth pavement and grooved rails the wear is reduced
to a minimum where the street is of sufficient width
to accommodate the traffic without necessitating the
driving of loaded vehicles along the track.
In the case of narrow streets or rough side-pave-
ments the use of the track for hauling heavy loads
causes the cutting of the pavement upon the outside
of the track, due to the gauge of trucks being greater
than that of the track. This is especially the case
where, owing to the use of side-bearing or centre-bear-
ing rails, the flange grooves are wide enough to permit
the wheels of trucks to enter them.
Where cable roads are used ties are not employed,
but the whole structure rests upon the yokes, which
CITY STREETS. 211
pass under the cable conduit and sustain the rails upon
their extremities. The conduits are usually built of
concrete, which is also used for the base of the pave-
ment, so that the whole structure becomes practically
ART. 83. TREES FOR STREETS.
It is always desirable, wherever possible, to have
streets, at least those devoted to residential purposes,
lined with rows of trees upon each side, both for the
purpose of giving shade and to add to the beauty of
appearance of the street.
The most satisfactory way of arranging trees is usually
to have a tree space between the sidewalk and the curb
in which the trees are planted in a straight line along
the street. Sometimes in very wide streets a tree
space or parking is arranged in the middle of the street,
with a driveway on each side. Trees should be spaced
in the rows at such distances as will permit each tree
when fully grown to spread to its full natural dimen-
sions, which usually requires, for trees ordinarily em-
ployed, from 25 to 40 feet.
The selection of the variety of trees to be used for
this purpose must of course depend upon climatic
and local conditions. Those which rapidly attain their
full size are usually to be preferred. They should
have a graceful form and make a good shade, but the
foliage should not be too dense. Evergreens are not
generally desirable for this purpose. Where there is
plenty of room for their development the large-grow-
ing varieties with light foliage are handsome and desir-
able. The size, however, must be suited to the space,
212 A TEXT-BOOK ON ROADS AND PAVEMENTS.
and upon narrow streets, or where the trees are to be
close to the buildings, they must be of small growth.
The ease with which the tree may be grown and its
liability to disease or to be affected by the contamina-
tions of a city atmosphere must be considered, as the
conditions under which street trees must be grown are
not usually favorable to their best development.
It is desirable, especially in cities of considerable
size, that the planting and care of trees be under con-
trol of the municipal authorities. Trees may then be
set with a view to the best general effect upon the
street as a whole, the selection and planting of the
trees may be properly done, and the trees after plant-
ing may be systematically cared for.
ART. 84. ALLEYS.
The pavements for alleys in cities are constructed in
a manner similar to those for streets. Cobblestones,
block-stone, brick, and asphalt are commonly employed.
The maintenance of alleys in good condition is a
matter of no less importance than the maintenance of
streets, although it is more likely to be neglected. It
is of special importance that the pavement of an alley
be impervious, well drained, and easily cleaned.
The surface drainage of alleys is secured either by
forming the section as in a street, with a crown at the
middle and gutters and curbs at the sides, or, as is com-
monly preferable with narrow alleys, by placing the
gutter at the middle and sloping the pavement from
the sides to the centre. Where the gutter is in the
middle it is common to make the bottom of the gutter
of a flagstone 15 to 18 inches wide. Fig. 43 shows a
centre-drained alley with block-stone pavement upon
Where the pavement is cobble or rough blocks it is
desirable also to form side-gutters of flagstones in order
to promote ready drainage. Such construction is rep-
resented in Fig. 44, which shows a cobble pavement
on a gravel base, with curb and narrow sidewalk.